1.5 inch wall thickness? MDF or plywood? If you are planning MDF, you might consider plywood for the substantial weight savings. Double baffle 3/4" plywood, with 3/4" for the rest of the box would net you a little under 5 cubic feet while still retaining the outer dims. And it would weigh less than half of what an MDF box would weigh - probably more like 1/3. Much easier to move.
Just some things to consider - most people build extreme SPL boxes using drivers with very high Xmax out of 3/4" plywood and have no issues provided the internal bracing is properly designed. Though I feel a lot of designs go extreme overkill on bracing. Some sacrifice a large amount of internal volume for no real world gain. Bracing is important, but the law of diminishing returns applies in the same manner as increased wall thickness.
Back to the isobaric discussion though - You had asked initially about 2 open baffle 15s, and judging by the depth of your current design, it appears you have a depth limitation, not necessarily width and height?
You also mentioned a passive radiator (PR
)? What are you doing with the PR
? Are you thinking isobaric 15s, face to face, with a single 15 PR
It would help immensely if you told us what your plan is for this box, and any applicable limitations. There are much more knowledgeable forum members here than myself that might be able to help you accomplish your ultimate goal with a little more information from you.