hi,
usually there's suppose to be a big metal handle in the back angled section.
you could put let say a few layers of bike tubes with zip-tie holding it to support the back of the amp and give it some grip and cushion and put 2 new rubber feet on the front of the amp by using the original hole of the present rubber feet. how's that.
or use the handle to support the plywood in the back?
I would have to see one cab in real to make sure it fits but maybe you got some hints to start?
some pics from the side would help to better know the depth of the cab vs the amp...
perhaps some beta bass head users could help you better on this because I never saw one in real so it's hard to guess the size of it.
but I'm pretty sure you'll find a way, you're on the right forum to ask!
you could always post your question in a new thread with some pics, there's a lot of helpful people here!
good luck!