I was just thumbing through this thread--
Really nice work here! I'm not an attachment method expert (or any kind of expert, for that matter),

but I was thinking,
1.) If you had the two straps attached to the pack, hooked on to your back plate, you could have one centered or two parallel downward-facing clips that could hook on to your belt so that all the weight of the thing isn't
all on your back plate?
And/Or
2.) If you cut two slots in the back where you want he straps attached, you could probably feed the end through one slot and out the other, but if you do that, I'd suggest leaving room between the two. I don't know how strong your material is, as I am a costuming noob (see signature), But I'd imagine that the closer they are together, the easier it'll be to bend or break in that spot.
The way I'm imagining it, if you have one slot near the top, corresponding to your D ring location, and the other slots halfway down, it'll be holding the pack from the middle, instead of the top, and won't be dangling or move around much. I figure, the more distance between the two anchor points, the more solid it'll be. But I am also not a physics expert.

This is a great WIP, vod!