I tried the velcro on the bottom of the pan and it worked well for a while.
after repeatedly getting wet either from urine ( Gadget is having Litter
training problems) and from cleaning it lost its stick.
 
I came across a small metal clamp which I now use to fasten the high end
of the litter pan to the side of the cage.  It has been working well so
far.  Gadget is unable to move the pan, although she tries very hard.
[Posted in FML issue 3099]