You don't need the box on the left to drop in a 10". There is a really good DIY write-up on here that explains how to fiberglass the left-hand well. I'll see if i can find it for you.

EDIT: I couldn't find the exact DIY i was looking for, but this should give you some ideas. I'll keep looking.https://www.audiforums.com/m_420235/tm.htm