I am not averse to some elbow grease. I hollowed out relevant areas in the housing of Germaine's stellar First Order SE-44C to make room for a bead tilt switch and battery compartment (battery held in with the rear block when it's screwed in place), and perma-mounted the sight rail to the housing so the wiring for the LED could just hook up direct instead of trying to fiddle a spring-connection. Now the LED is off while the blaster is riding on the thigh bracket, but comes on as it gets brought up to level.
I've been practicing field-stripping my Armorer Works airsoft pistol, so as long as your upper fits reasonably well over the stripped grip/trigger frame, I can do whatever precision butchery I need to inside to get the desired result. Heck, I'm making a partial-duplicate magazine (as tibanna gas cartridge) with a base-plate to blank off the bottom of the grip. Going to ludicrous extents for my costumes has never daunted me.

Looking forward to studying your model(s) and planning my approach...
ETA: Ooo... The season one version. Some reason I thought this was the season two version you posted early work on on the previous page. I like that one, too, and am totally going to do one, but it's the S2 I'm needing most right now. Were you going to make and sell those yourself?