Help with enclosure design

PE website states that the mounting depth on the UM18-22 is 9". So, even if your O.D. on the box is 15", and you use 3/4" sheet stock (MDF, Ply, etc), and you double up the baffles, you're still at 12.75" of depth I.D. That gives you 3.75" of clearance from the back of the motor assembly to the inside of the rear panel. That's plenty. And if you go one step further towards Worst Case Scenario for mounting depths, and countersink the driver frame into the outer sheet of the baffle, you only lose another 3/8" to 3/4" of clearance, leaving you with a minimum of 3" clearance. You're good.

If you use the UM15-22 instead, you buy another ~1.5" of clearance, as its mounting depth is only 7.55".

Point of Interest: With a driver that moves as much air as these do, and with as much moving mass as these have, a single layer baffle and panels less than 3/4" thick are not advisable. The UM18-22 is throwing a pound and a half of cone around on the surface of that box... you'll want as much mass as you can get to dampen that.

four 18's
two boxes with middle divider

external 49 wide X 15 deep X 27 high, side by side = 98 x 15 x 27

internal each chamber
23.375 X 12.75 X 25.5 = 4.4 cubic feet - 0.26 driver = 4.14 - bracing = real close to 4 cubic feet per driver

will work fine. make it taller if you want more air space. At 49" wide you can use entire widths of cabinet grade MDF. Go beyond that and your cut list will start getting inefficient unless you plan well plus it's harder to rip sheets of wood the long way.

Look in the other threads on this subforum for "quad ultimax build", I did this same thing except deeper and shorter. It's the same width and air space.