DIY CDP Platform...

There are many choices depending on what you are trying to achieve (e.g., isolation, damping, resonance transfer). I recommend thoroughly searching the archives here and at audio asylum for more discussion on this and information. Your best solution could depend on the characteristics of the rack, table, shelf, etc. that the platform will be placed on. Some options that come to mind range from a sheet of mdf, layered mdf, an air bladder made with an inner tube placed on mdf then a floating mdf platform above the tube (sides can be added to dress it up), to a symposium clone consisting of thin aluminum sheets on the top and bottom, adhered to mdf, sandwiching a layer of presentation board in the middle (search gatorboard). If the spikes are threaded, I recommend using inserts into whatever you attach them to, so you can use them to level the cdp. Have fun.

Buy a pre cut piece of granite.........

If you can't afford granite or marble, Corian (synthetic marble countertop) works very nicely.

Go online and order a Maple cutting board," Boose Blocks ". Rock Maple 2" thick and stable. About $75. You already have the feet.Should work awesome!

Can I suggest a sand box.Picture a drawer.Make a shallow one with mitre corners for finish look.3/4" sided ply works very good.Put a bottom.Attach everything with glue and nailgun.Create one floating shelve,by cutting a slightly undersized plywood the size of the box minus 1/8" overall.
You could go with double compartments.One holds sand the second on top holding a bicycle innertube a little under inflated.Your spikes go on the bottom.
Best of luck
George