Dan, I personally think that you'd be better using a rack to support the weight. I haven't used a Bagman myself but there are lots of reports of a tendency to bounce - it wouldn't be a problem with a smaller bag but I think it would be quite disconcerting with a full Camper, and must put quite a strain on the saddle rails. If you do get the Bagman I would do without the support loop and either let a rear rack take some of the weight, or do without, fasten the bottom retaining strap to the seatpost and put up with your thighs brushing the bag as you pedal.
Of course back in the day when large saddlebags were more common, people didn't bother with quick release, but strapped them up snugly to the saddle and seatpost.
It's possible, depending on your set-up, to bodge a QR solution, with hooks such as
bimini clips. Basically, you remove the sprung retaining bit, then put the bags straps though the slot so that the open part of the hook faces to the rear. You insert the hooks into the saddle loops, approaching the loops
from behind . Then secure the bottom of the bag either to the seat post or the turned-up bit of the rack if it has one. Because the bag hangs at an angle, the hooks can't jump out of the saddle loops. It does depend on your particular geometry but I'd have thought that with your metal-working skills you could make something to fit. (Those bimini clips are bigger than they need to be.)
I sometimes hang a Barley like this, using
gaiter hooks zip-tied directly to the wooden dowel. It works really well as a QR except that the gaiter hooks are aluminium and wear quickly - I keep looking for a better solution