Best way to seal a big cabinet

[shilala]

Those cabinets are not very structurally sound. They're nice and they'll work good, but you have to be cautious with them.

First thing is that you have to have them sitting on a solid surface. No carpet. Otherwise they'll rack and you'll never get a seal. If it must stay on carpet, an oversized piece of 3/4 plywood underneath the entire unit will suffice if you can make it stay level once the unit is on it.
All that stuff on top, it's gotta go. That weight will cause the unit to rack out of square and the door won't seal. It's not meant to have stuff on top of it, and it can't support it. It's not designed to, and it's not engineered to have things stacked on it.
Try to balance/spread the weight of your boxes and stuff evenly in the unit, too. That's important.
Once those issues are addressed, you can let it settle for a bit and see if she squares back up. That may alleviate some issues, then you can march on from there.
Right now you're in "user caused error" land, my brother. Get that ironed out and it'll likely fix itself.

If not, there's all sorts of fixes that come next, but you have to start with a solid foundation.
Good Luck, my friend!!!