Sorry for not responding earlier, but I seriously suggest paying a visit to the audio asylum boards. There you will find a recipe for DIY platforms in numerous shapes, forms and sizes, but the predominant ingredient is PEF, aka closed cell polyethylene foam. It is quite a hard foam, the only way I can describe it is somewhere between a mattress and rubber :) however it does wonders. The rest of the platform depends on budget, availability, aesthetics etc. Probably the best design is a sandwhich which consists of the following:
- 1/8" aluminium (mainly for looks)
- 1" MDF
- 2-4" PE Foam
- 1" MDF
- 1/8" aluminium
You can play around with the rest of the sandwhich but the main thing to keep there is the foam. Then on top of the sandwhich place your speakers on their spikes.
Cheers,
X