If you find a solution please let me know.  I have a similiar problem.
I had a water bed.  Went through three mattresses and gave up.  I got a
regular mattress and put it in the waterbed frame.  There was a 3" gap at
the end which I filled with paperbacks.  There is now a hole there.  My
little monsters would push the books out to get into the hole.  I then
took a board and covered the book area.  They pushed the board off, pushed
the books out and climbed in the hole.  Now I have their food container on
the board which is on the books which is on the bed.  That seems to be
working.  I am thinking about making a permanent fastener type thing with
hinges to cover the area but still make the books accessable.
 
Unfortunately, Ornery (and yes, he is) has decided to make an alternate
route into the bed.  I now have a hole on the top of my mattress.  I
covered it with duck tape, a mattress pad and a comforter tucked into the
sides of the bed.  We're okay if I'm actually on the bed, but he's managed
to tunnel under the blankets once.  He was digging into the comforter
last night as I was reading on the bed and couldn't understand why I was
getting so upset with him.  I'm considering getting a big board to cover
the bed when I'm not in it.  I am also going to get a dig box with sand or
something to give them an alternate place to dig.  Maybe that will help.
 
Persisistent little buggers, aren't they :-)
 
Debbie
[Posted in FML issue 3594]