Honestly, I don't know if anyone could tell a difference if the inside corners of the box were rounded or not. In such a small volume, theoretically, the air should act more like a piston than a wave. Basically the pressure would be equal or god dang near equal at any measured point within an enclosure at the same point of the wave. While, I'm not saying it's a waste of time, because I've never seen it in practice, I'm just not sure how much of a difference it would make.
About the MDF, yes, it's good stuff and it's cheap. The most improvement of the enclosure will be gained in its stiffness. I've heard talk of marine grade plywood and even baltic birch having a stiffness that is roughly 2 times greater than MDF. Mr. Dan Wiggins from the teampamp.com forum even had a post saying that marine grade ply is good stuff. I've never used this stuff either... but, jeez, if it really is 2x or 2.5x as good as MDF as many people say, it's totally worth it! You'd be getting a cabinet that is 1.5" or greater in thickness compared to MDF.
DEQ-P9 (30band eq L/R, time alignment, 36dB/oct 4 way crossover)
Soundstream Tarantula (for subs)
Precision Power PCX-4125 (for horns and midbasses)
Image Dynamics CD2 comp mini horns
Scan-Speak 7" 18W/5535A midbasses
Subs still pending.