Any stable insulated material should be sufficient for the motherboard to rest on.
Ideally, an old motherboard tray with standoffs still in place would be great for screwing down the motherboard. (Although securing it to the rest of the frame would be another matter.)
A piece of wood should do the job, however.
Just my
BTC0.02, I would never purchase a pre-built mining frame. The cost of the frames is almost always disproportionate to the material cost and the time it takes to assemble them. (Plus the IKEA effect from assembling your entire rig on your own.
)