Although it's not movie-accurate, I glued (reinforced with rivets) 2" elastic between the butt and codpiece. It's just tight enough not to fall when I wear it. To put it on, I just step into it like a pair of shorts.
You could use some elastic. Attach a strip of elastic from the cod piece to the butt plate on each side. Make it taught enough so that the tension will hold the whole shebang together. The strech in the elastic will allow you to pull them up and on like a pair of plastic undies
I used the bolts I got from Rodann, and bolted mine together, and the pieces would bow out a little, so I put velcro on both sides to keep 'em aligned. I got no hips, so I can kinda just wiggle my way into the cod & buttplate, but I can undo 'em if I want or need to
tisk....tisk...Jango needs no steenking butt plate!
Actually....Boba would fly much better without the butt plate/G-stringy thing attached to the armor wouldn't he?
Just think of it, that's one super wedgey when that jet pack fires off. Maybe that's why he didn't fly so well in ROTJ and the blame was pointed at Nitko.