Here's an update. I completed what I will deem the cheapest hushbox I could come up with. It's a plastic walmart bin flipped upside down and mounted to the ceiling, lined with some strategically placed linacoustic. There is a flexduct that discharges the hot air from the projector and is sucked out via inline duct fan from the adjacent room. Here are the pics:
This is the bin mounted to the ceiling with the holes cut out for the air. I used a 6" diameter plastic cylinder mold (mentioned earlier) to connect the flexduct. It's the white cylinder sticking out of the side. The projector is not attached to the mount, which is just a 'monkey man' type diy. So, the top of the bin (actually the bottom but top as it's mounted) has a hole for the penetration into the 'hushbox'.
Here is another shot of the same but a slightly different angle.
Here is the box with the flexduct attached. It's just typical 6" diameter flex duct (not acoustic). The projector has been mounted inside the bin and the flexduct actually goes behind the 'hushbox' after it bends around from the outlet. You can see the duct goes to the penetration above the equipment closet. It actually has a small hole where it goes through the equipment closet to help pull some of the heat out of that area as well.
Here is the 6" cylinder (actually a concrete test mold) I discussed earlier in this thread. I used a few of these for my penetrations with the bottom cut off.
Then I just put the linacoustic back on the back wall and covered the duct with some black fabric so that it disappears.
There is linacoustic (1" thick) inside the box. I put 2 layers on the bottom of the 'hushbox' (again, upside down bin, it is actually the lid). Here it is in action.
This was very inexpensive (and it kind of looks that way - at least when the lights are on). But it is effective. The input (hole in the back) is right next to the projector input filter/fans and the discharge on the side is right next to the projector fan discharge. I did the paper trick to check that there was good air flow - pulling air outside of the box. The paper trick is just me taking a piece of paper and placing it over the hole in the 'hushbox' to make sure it sticks - and it does quite well. And, it appears at least, that the air is not being short circuited from the other input holes. Just thought I'd share. I may try to wrap some fabric around the box to hid the fact that it's a plastic bin. The fan a 6" diameter inline duct fan, Suncourt, and discharges into the adjacent room which has the furnace and water heater. I just plugged it in to a wireless remote extension. I have not done anything, yet, to automate it with a thermostat. For now I just make sure the fan remains on well beyond the cool down time for the projector after shut off.
It's very effective at removing the hot air from the room and I have another inline fan to circulate air back into the room via the new penetration mentioned in the previous post. For now I have just put a register (vent) over the hole.
I'm always very inspired by all the nice wood hushboxes I see being installed but I just don't have the skill or budget for something like that (wish I did and maybe someday). This'll do for now!