Sandbox Design Advice

I understand the recommended design is to build an open wooden box plus a loose lid (possibly granite) which is slightly smaller than the internal perimeter.

Fill the box with kiln dried sand, float the lid on top of the sand and then component on top.

Heavy duty sorbothane feet for the box.

There were a few design details i couldn’t find on the forum and so pl can anyone advise on:

1. what size should the gap be between the lid and the perimeter?
2. how deep the sand should be for a 52 kg power amp (114lbs in old money 😀)?
3. i am going to use a carpenter to make it for me (pine?) and would like a professional finish in black - any suggestions? 

thanks very much - advice is much appreciated.