The easiest way would be to use some sort of sticky tape/ velcro. I don't know how stable it would be, or even how long it'd last though.
I've got a case for my portable DVD player that I got at Best Buy. It came with several straps that can be wrapped around and adjusted, allowing you to tie the case to pretty much anything. It's got clips on 4 corner of the case...
this is similar
to what I've got
it can be a bit of a PITA to tie everything up, but it does have the advantage of being something that can be removed whenever you desire, and not mess up the interior