I made a two layer version out of the schematics that fly around on the interwebs because cheap:
Couldn't get them to work reliably yet because of the whole PCB thickness thing.
If you wiggle them around before use and put on rubber bands they are ok for a while.
On the one hand, these are almost certainly an investment. A IIfx with 128MB will command a much higher price than one with merely 32MB, and surely the price-difference will be greater than the $240 required for the new RAM.
Sets of 4 are a good idea. 64MB RAM in a IIfx is almost obscene, but the extra 48MB will make a useful RAM disk. It also allows more apps to run on Mac OS 8.1.
I do want to buy some though.