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.