I too am sorry you are having to think about this. In my area it is illegal to bury exotics. I personally have yet to have this issue personally but a friend of mine came up with an interesting solution that I can share. He bought a concrete mould and placed his bearded dragon into wet concrete that formed the underground footing for the bird bath (the mould he bought). He added a small plaque to the fountain in memory. Although not strictly legal the concern about exotic pets and native species contacting each other is moot.

I thought it was a very elegant option regardless to laws in your area, add that to the other beautiful and tasteful options already suggested.