I'm getting ready to finally get to putting up the star ceiling, but I'm still not entirely sure what to make the darned thing out of. I've gone back and forth on a few ideas, but right now I'm thinking thin plywood covered with black cloth. Suspend this with cables having the anchors hidden behind the cloth.

I don't want to use 2x4 suspended ceiling panels as I'm trying to avoid seams. Edge support also doesn't seem like it would be sufficient as the middle of the panel would likely bow out.

Am I way off base here? Any pointers?

How big of a panel were you thinking of making? Since your considering plywood I suppose 4x8 or smaller if that is the case you can perhaps use insulation board. I was able to use 1" thick insulation board Lowes.
Here's a link to pics on my thread.
http://www.avsforum.com/avs-vb/showpost.php?p=13666672&postcount=23
It shows you the solution I'm planning in order to secure due to the gap that I need above for the light units. I will trim off the frame from the sides and then secure from underneath once the panel is inserted with additional trim. It is very light, yet stiff eneough to hold it's shape with the fabric being streched over the panel. The fabric I bought was more stretchy than I would have liked so I couldn't use adhesive but what I did was add eyelets to edges of the frabic and basically laced it from behind after I laid out my star design. Doesn't look pretty from behind but it's held in place no problem. I was then able to poke the threads from behind with an oversized needle from walmart, no drilling or needing to heat the needle just a little pressure to poke through then I used needle nose pliers to finish pulling through. Glued in place once all the fibers were pulled through.

In the accesories area there was a star ceiling thread were I posted some other pics. On this this link you can see actual pics of the board before being covered.
http://www.avsforum.com/avs-vb/showpost.php?p=12217871&postcount=59

This is the link to the actual thread

http://www.avsforum.com/avs-vb/showthread.php?t=911313&page=2&highlight=star+ceiling

Depending on the height of your ceilings if you are wrapping your panels in black fabric you probably will not even see the seams with typical theater lighting.

I used 1/2" mdf panels with black velvet. The finished height is about 8'6" and the seams aren't noticeable. The last page of my HT link shows some construction pics.

I used framed masonite. I covered it in black Celtic cloth but would not recommend using it because the cloth is directional. I think Velvet is your best bet. The screen lights up a room and you will see seams if you use just black material. I have some shots in my thread link included below.

How big of a panel were you thinking of making? Since your considering plywood I suppose 4x8 or smaller if that is the case you can perhaps use insulation board. I was able to use 1" thick insulation board Lowes.

Oh sorry, that would be valuable information. The area I have for the ceiling is a tray above the seats. Works out to 3 4x8 panels. Height is 9ish feet.

There's straight up styrofoam board which I wasn't sure about being able to work with, without it flaking all to heck. There's also some stuff that's a foam with foil cladding on both sides where it looked like it might be difficult to get three that aren't wavy or creased. Those were my top two contenders, but I'm wondering if thin plywood might just be easier to deal with.

The Dow insulation boards that I used don't flake. They have very clean edges and can be placed next to each other for a clean seam. The material does easily indent if pressed upon in a small area, but not if laid across 2x4's on horses. Nothing shows thru on mine and the material is very taught, even if you glue I think you should be fine for no indentations to show thru. Good luck whatever you decide.

I'm wondering if thin plywood might just be easier to deal with.

A 4x8 sheet of plywood that's only supported around the edges will sag in the middle over time. It could be braced to prevent sagging, but unbraced it will sag. The other potential problem is finding a sheet of plywood that starts off reasonably flat. If you find a flat sheet of plywood (of any thickness) at a big box store, go buy a lottery ticket because it's your lucky day.

Big box stores usually stock two different insulation board products. One of them is styrofoam-like and prone to crumbling. The other is an entirely different material that's usually gold colored foam covered with foil or mylar. It's more expensive but much less fragile than the styrofoam-ish stuff. That's what I would use. Flat, cheap, easy to work with and rigid enough to support its own weight without sagging.

I was all set to use the styrofoam insulation panels with suspended ceiling rails and I noticed that the labeling said to keep away from open flames. Hmm, the real drop ceiling tiles are a flame retardant while the styrofoam is a flame accelerant. Nixed.

There's some other stuff that's foil-cladded foamy stuff that looks similar to Great Stuff when dried. That also said it burns like mad.

MDF is crazy heavy, and I'd rather not go that route. I think I might actually go for actual 2' x 4' ceiling panels. I cannot find these mythical Dow Insulation boards.

Edit:

Amongst the 2' by 4' panels, I'm looking at the ones that are fiber glass panels with plastic cladding on one side. They might very well be tougher to work with for pushing fibers through, drilling and whatnot, but the other ones are stamped with CANCER CANCER CANCER all over them. It says to wear a space suit and operate in a hard vacuum with a good solar wind to blow away the particles when slightly cutting them just a bit or else you'll get CANCER CANCER CANCER. I figure drilling with Dremel would be bad.