Making Fluid bed cups is really simple and easy.
First buy some PVC and some Knock out test caps of the correct size from the hardware store. The test caps are in the same aisle as the PVC and cost ~35 cents each.
Cut your PVC to your preferred length and clean up the cut end. Insert the test cap and knock out the center with a hammer and discard.
Remove the remaining ring. Place your membrane over the end and reinsert the ring from the test cap. Viola a new fluid bed cup for cheap.
Buy extra test caps to cover your fluid bed cups when leaving paint in them.
For membrane material you can use vacuum cleaner bag material,Tyvek(housewrap or USPS envelopes)label side up,Brown bag material,printer paper of different weights, etc.
I learned this from a TU member and have been doing it this way for years.
I also bought some Fluidizing plate on EBAY and made some cup with it using a hole saw in a drill press without the centering bit then hot glued them in.
But the other method works great and is much easier for most people.