Step 1: The bag
Step 2: The materials
When you're satisfied you can draw the layout on a big paper sheet, so you can follow it to cut, bend, glue the pieces of frame. This frame could be made of wooded boards, probably plywood, or made, as I did, from plastic baseboards. Indeed that material is often very malleable if heated up, and you can cut it with a pair of big scissors. Pay attention to find the right material, not all the baseboards are good.
I've glued each piece with hot-glue, then you probably want to paint everything black.
You also need some cushion material, as you see in the picture I've cut each shape to fit the right compartment.
Step 3: The frame
The two "arms" are intended to keep the frame steady in place pushing on the internal corners of the bag.
The compartments of the lenses have on the bottom edges some foam pieces with triangular section, so to obtain a rounded stand.
I've also cut apertures to avoid that the viewfinders interfere with the plastic frame.
Step 4: The stuffing
Step 5: Almost finished
Yes, it' not very nice to see, but I'm sure you'll be able to make a better work. I should have painted black the frame before gluing the foam sheets...
Step 6: Close the gaps
The other method is simpler but not so effective, only add glue on faces between the two foam pieces and close them.
Both methods will be anyway better than leave everything open as I did. I'll soon close everything with the first way.
Step 7: Fitting
Step 8: Bon voyage
I've inserted a big very soft foam in the big square pocket to have a comfortable pressure on the back, although the backpack already has a very good padding, so if you need more space you can avoid additional foam.
It's time to wish you bon voyage ;-)