Most of the energy transmitted to a box is mechanical coupling from the driver mounted into the box. Internal air pressure -- even in a sub is, at max, 1/3rd of the energy and particularly in a vented box, much less than that,realize that a LLT is one of the least demanding subwoofer enclosure applications, as the force created by the driver is spread out over such a large area (large enclosure), meaning lower pressure than say a small sealed sub using the same driver.
A larger box means larger panels which are easier to excite... it should be well braced and ideally no 2 subpanels should be the same (in practise thou you will probably have pairs of identically shaped sub-panels on opposing sides of the box). You do want to avoid subpanels that have dimensions close to one another. Given a panel, the best brace orientation is the one that divides it into panels most removed from being square (ie in a floorstander a shelf brace is much less effective than a longitudinal brace. I'd try for sub-panels less than 12" (you only need consider the smaller dimension in a rectangle)