I don't want to steal the show but my lid arrived earlier today so I took a few minutes to try it out. It worked very well on my Mountain Warrior bag.
The female buckles on the lid front clip right up with the male buckles on the bag. I worried they wouldnt for half a second then remembered how much Kifaru does with the military and realized they'd likely clip right up and they did.
The back took a tiny bit more work. I found a buckle in my junk drawer and it worked too, but only one so I have to remember where I got it. I grabbed the female side which was didn't have an adjuster on it so I grabbed a three bar slider and made up what is shown in the photos below. It works pretty well and I didn't and won't hack anything off the pack.
Close-up

Thanks again for the idea. This frees up a few future bucks for some pockets or other stuff sacks.