sub box starting to come apart

I would do it different. Someone up above in this thread has keyed on it already. Reinforce the box. Take out the screws, redrill for longer ones and hand tighten. Do not over tighten. Use some more glue in those places. Add bracing on the inside to keep the large surfaces of the box from flexing, the more the flex, the more the pressure on the joints. Add fibreglass to the seams on the inside, 2 layers is more than enough. At least 2" to each side of the seam, so you will have strips 4" wide and who knows how long. This will make the seam both airtight and far stonger. Rough up the mdf on the inside of the box in the seam area so that the resin and matting will have more to stick to. Next, if your sub box is not mounted properly, get it mounted properly. If it is moving and crashing around, it is only a matter of time before it comes apart or takes your head off in a accident.