Good call Ill check it out if I can find some, they do have some white wall press board paneling that people use for dry erase boards, not the real deal though, ive seen old used big classroom ones sold on craigslist sometimes.
I was at home depot last night and checking out different materials. The 5mm underlayment shot up to over $16 a sheet which is rediculous, but then again more people doing flooring in the winter, while 9/16ths OSB is $9. Usually vice versa in the summer months. 3/8 or 1/2 inch plywood is only a dollar more, but all their stuff is pretty warped.
They have white vynil coated 1/4" mdf @ 22 a sheet which looks nice, but not very rigid and kinda heavy.
The best material looks to be tempered 1/8" press board, cheap at $9 and doesnt flex much, but i started thinking about checking for other stuff around first. Its #1 choice so far.
The strips i'll use inside the boxes to channel the air should make the fixtures less flexible anyway, another reason im doing it that way.
I may just use a thicker 1/2" panel for the bottom with the SILs attached though, and do a little framing and use thinner paneling for the rest of the box cover.
Ill use thin weather stripping/felt to seal the access panel, and glue everything else so its all air tight, including some rtv silicone on the SIL pcbs right where the pins from the driver connect in the center.
The weather took a turn for the worse last night and been rain/snowing, or I woulda said heck with it and went back today with the truck anyway. Truck bed full of wet snow no go..
Oh yea and they have the rectangular metal ducting. 3ft long half sections for $7, and 5' sections for around $10. Need two halves to create the duct, but they are hard to take back apart if you need to replace anything. Unless you reach in from the end. I will use them for another project in bigger spaces later this year. Cutting the holes wears out hole saw bits quickly too, but man they would look nice and be light. I hate paying full price for the register box fittings on the ends too, love finding them brand new at the re-store for a few bucks each, but that could take awhile. I dont want too much ducting in my small room. But long rooms for sure.
I'll be less than $20 for the boxes if i use the tempered utility press boards. 40 or so if i cover them with Plastex 1/16" plastic wall paneling sheets (like cheap FRP), and a coat of chrome spray paint on the bottom panel.
i dont mind spending money and using higher end materials on mine, but like to keep the costs low so others might want to try, or my friends might want me to make a few dozen boxes. I could have been mass producing simple fixtures with base sockets, and made 100 of them by now lol, but new designs are fun to figure out!