New project - surplus saddlebags

Here's my latest project - I want something for the W650 that I can carry some luggage in, but rather than go out and buy a luggage system I figured I'd "fabricate" one. I have 2 of these old army rucksacks, so I've set about modifying the first one by removing every bit and bob that's not needed. I'll then send it off to a pro to add some bits of leather (like on the back for abrasion, and on the end of each strap) and join together what needs to be joined together. I'll probably then scotchguard the whole lot as well.

The design is a basic throwover saddlebag - the bits at (1) will be cut off and the straps joined to the other bag's straps, to be able to chuck it over the seat. I am not sure if this is sufficient to hold it in place though - there are rings on the back as well that will stay and these can attach to the frame via a velcro strap or similar. Can anyone advise me on how normal saddlebags work?

I'll probably retain the buckle system at (4) but am considering removing the compression straps at (2) and (3) - what do you guys reckon? Surely I won't need these? If I do keep them, I will shorten them considerably. Every little ring, strap, etc that's not in use is being removed to simplify it all...

Your thoughts?