I think most people with a 15" go with the ported cube.
Is that because they dont have another option other than a cube? I love the thin profile of the stonehenge. Not everyone has room for an 18 or a deep cube. Or some people have 15's sitting around collecting dust and want someting a bit different. I have no problem making my own thin profile sub box but flat pack kits are so nice, well thought out and designed.
You'd only need to cut one panel yourself to make it sealed.
I seriously considered doing just that. I also thought about getting the flatpack and cutting it down to 24"x24", which is much closer to my ideal size (and the depth is perfect). That got complicated pretty fast, and at that point it seemed like a lot of work just to keep the cool bracing system. Thought maybe it was an easy mod to the CNC program, and all of the braces are the same, etc. etc. - probably ain't gonna happen though, with all the other cool stuff Erich has coming out these days...