I wouldn't trust that local shop as it seems they are not sure what they are doing to be honest. I have put triple the power you have to a single layer enclosure with proper bracing and a double front baffle with on issues. As for fitting it would slim down 1.5" off any side that was doubled.
You will notice a decent loss of mpg and by mid summer when prices are worse you will regret it lol.
I told you before, you went so far over kill with that enclosure it was not even funny. It works, so no biggy, but it was not over kill it was just a waste of wood/time when you actually look at it from an outside perspective. However, if you like it...then who cares....