Optimal Enclosure Shape For The Mid Section?

I just inherited a RAW RA8 speaker kit that is WWWMTMWWW ported.
The MTM part of the speaker is sealed with an inner dimension of 15.5"X7.5" rectangle.
What is the optimal inner shape for the Mid Bass unit? Should I fill the corners with epoxy?
Should I keep it sealed or cut out a hole in the middle and put a PVC pipe in it to make a T-Line out of it? How good does a Mid sound as a T-Line? And one last question about baffle round over’s what is the point of diminishing returns and how thick would the baffle and sides have to be to get it there?

Thanks Ben