Best material for inexpensive amp stands

Tvad's suggestion is great (as usual).
Another option is Bamboo cutting boards.
These come in many sizes (small to very large) and
can be found in the housewares section of major department stores and also Linen 'n Things.

i glued together 2 pieces of mdf and finished them off nicely with sanding sealer and krylon paint..

heavy brass cones for feet (you can get the screw in inserts from a woodworking shop (also of brass)

Little work on my part, but relatively nice looking and accoustically dead. You can go a little further by applying some sound deadening material to the bottom (out of sight). Something like Dynamat.

How about putting cork matting between the amp and the platform to absorb the vibrations ?

What is the floor type , wood on trusses with a basement , vinyl , carpet or cement slab ? What is the traffic situation on your street ?
A cement slab on a rocky grade beside a major busy street can receive vibrations from heavy vehicles on the street .
As can an old rickety wood structure .
Ever been near railroad tracks when a train goes by ? Ever had a record skip when you walk by the turntable ?
It all makes a difference on the way that you do things .
Instead of draining those vibrations away from the amp you could draw them to it !
Just as important as the material used for the base .

Good luck .