I think it'd be MUCH less complex to just design the kit to hold its OWN mechbox, I'd suggest the V.3 as they are more durable than the V.2 and unlike the V.6 upgrades are more readily available.
I'd forget about trying to mold it to fit around a "donor" gun and make this more cost effective to the people buying it. Build it so that you can get the kit out with no internals to go with it, and then market an internals kit. That way you can sell the kit where a consumer such as my self can either get the the internals kit, or custom build our own boxes to fit.
Seriously the REAL costs to the M82's out there isn't the damn kit, mind you thats $1500-2500 for the kit ((last prices I saw)) and then you STILL gotta get a $400-500 gun to complete it, and thats before mags and batteries.
Also, if you use the V.3 you can have the motor cage around it, so that your grip can be much more compact.
And seriously, its what a few pins to get the mechbox to hold in place how much harder would it be to mold it to do this? And if need be a few holes to get pins to lock it in place. EVEN if your cost was $1500 for the kit at the end, you'd still see kits going out for those who would buy said kit elsewhere because of not needing to donate ANOTHER gun to finish the one you just bought. I'd try to aim your costs at the range of a good M249 off the shelf if you wanted to get maximum potential, granted as Droc said molds are not cheap.
While you're on the 50 cal's though, a good M2 Browning wouldn't be too bad of a market either

. I'd rather donate my truck to a .50 cal machine gun than my Colt to a M82 kit :P