You can get a "in-ceiling box mount" at Home Depot or Lowes that will make your mount sturdy and worry free. These are made for no attic accesss. They are found in the fan department. They are made to support fans, so, you know it will hold 15lbs of mount and PJ. The mount is made to slide into the hole and screws out creating force on each joist up to 24" away. The electrical box (fan mount box) then mounts to the mount. I got mine at Home Depot, it comes with a box made for fans. This means beefed up bolts and nuts built into the box. It costs ~$14.
Go to Homedepot.com and search for SKU: #398992. Click on the picture.
You can mount the plywood to the box or mount the PJ mount to the box, either way it is sturdy.
I used a plate made for outside lights with 1/2 inch threaded hole to attach to the mount, ~$2.00, 10" threaded rod, ~$2.00, floor flange mount 1/2 threaded hole, ~2.00. 3/4 " plywood, ~$4.00. Some bolts and springs, $4.00.
Maddawgmc gets the credit for the mount. The springs make this very adjustable. I could not believe how much difference a little turn made. I made a few changes, like the plate on the ceiling.http://www.avsforum.com/avs-vb/attac...483&fullpage=1http://www.avsforum.com/avs-vb/attac...490&fullpage=1http://www.avsforum.com/avs-vb/attac...449&fullpage=1