Within fiscal reason. I saw a PLOG with carbon fiber sheeting, ultra cool, but beyond my budget.

What else is out there? How thin of plywood could be used for example?

I want to make as small and light weight of a projector as possible to mount on my fairly low (7'4" ) basement den ceiling. I don't want a lot of extra space in the enclosure or wieght of materials.

You might look into a material called "Gator Foam" or "Gator Form" (not sure which is the official name). There are a few topics about this material, I'm sure the search function of the site will turn them up.

its gator foam. thin, light weight, and strong.

to keep weight down you could build a frame from 1x1's or 2x1's and cover it with 1/4" particle board or plywood. The thinnest I would go with just plywood or MDF is 1/2". I choose 3/4" since my box is rather large and I wasn't confident in the rigidity of 1/2" ply for a 40" box

A Haas style projector uses cross beams for support, I would think that would allow light weight (thinner) side and end materials.
I know countertop laminate sheets have been used for tops/bottoms on Haas style projectors too.

Couldn't find hardly anythign in LL forums on Gator-Foam, but it looks promising if it is somewhat heat resistant.

Anybody found any sources for GatorFoam?

another possiblity is stretching felt over a wooden or metal frame and then fiberglassing it. I made a subwoofer box for a car this way and it was lightweight and I could jump up and down on it. Only concern would be heat rating but fiberglass is so used I can't image theres not some heat specs out there

i was really interested in gator foam for the longest time. it is made by a company called alcan composits. i called them up and asked how i can get it and in the Los Angeles area they only had 1 distributor somewhere close to Whittier which is really far from Los Angeles.

but i called that dirstibutor anyways and it was rediculously expensive. they wanted like $80 for a 4x5 ft sheet and the minimum you could purchase was 8 sheets which comes out to $640 and a whole lot of gator foam left over! (and not to mention that a 4x5 ft sheet wouldnt even fit in my car so add another $100 for the uhaul)

so yah.. im back to looking for wood.. homedepot here i come

instead of gatorfoam couldn't you just use foamcore board from the craft store? or if you go to a fancy art store you can get this foam-core product that is sheeted with styrene instead of cardboard. and wouldn't something like coroplast or similar corrugated plastic be better than a foam-core product? you could also just use acrylic or lexan sheets and paint them from the inside (it looks really glossy then) or get black sheets. or sheets of ABS. all lighter than mdf and easy to work with. if i could go back i'd make most of my projector from a polymer product instead. if you have a local plastic store like tap plastics it is worth a stop to see what they have. they give you advice too with alot of free little pamphlets about working with certain products. kind of like a hobby shop for plastics and fiberglass

With a full rigid frame for support, I would think many things might be possible. The university in my city has an art store with foam core board. They even had 3/16 inch flat black foam core and black corrugated plastic.
My only worry would be heat resistance of the materials. I guess I used good heat shielding around the lamp, it would probably be okay.