I realize I would have to buy more rabbets and alter the lower part of a cut off box.
Not real sure what you are talking about here. All you have to do is cut off the bottom of the box to the desired specs. I think M. Bush has done this.
Meaning, I would cut the deep into two boxes, in the box that did not have the rabbets for the frames to hang on I would have to make a wood piece to hang the frames on, the rabbets are metal parts that sit on the wood piece the frames hang on. The metal rabbets make the frames easy to move off and on. Not sure if that makes sense, but does to me.
Are the metal rabbets used in the other countries? Maybe it is only here in my area. I don't know.
I will look at MB's site again. Probably I would do as you say, just cut the bottom of box to the specified depth. I'll check this out. Thanks. Great day. Cindi