How to make use of space where generator was?

Thanks for the quick replies. I was thinking of using a manufactured product to attach onto the frame but I'll take another look and it might not be necessary.

Building the box isn't my concern, attaching it securely to the frame is. I'll take some pictures this afternoon so you guys can see what I'm dealing with.

I am not saying no to a generator for the future but in the past with our class B we fared pretty well without one.