Interchangable baffle can be done.... I've done it several times. Instead of using wood glue when mounting the baffle, i've used silicone glue. You just need to be sure that the blue are fully dry before using the box. I used screws to bolt down the baffle. In fact, I just did one this weekend for one 15" Fosgate hx2. 3.6 cube tune to 35hz (3 - 4" areo's). My idea was to make it so I can use two 10's if needed.
I don't have any experience with the BTL but 5 + cubes seems a bit big for the 15" imo. I wouldn't personally put a 15" in a box bigger than 4.5cube unless it is what recommended. For two 12's it's alright. If you want to continue going interchangable baffle, I suggest to net around 4.5 cube. I think 4.5 is still good for two 12's.