Stitch welded and sealed on both sides. It's a jeep not a submarine. I'd bet they have more linear inches of weld than a set of tube sliders do. Im sure they are the single strongest pice of metal on this floppy unibody now and break over clearance is great.
As stated above if under stress it will be compression and I'm sure the unibody will fold before the welds break.
I opted for no legs to reduce chances of hang ups on the legs. It's a dd/weekend wheeler on 31's anyway. I'm not doing serious rock crawling with it. I haven't had any issues yet and the body has noticeably less flex in it.
I have 3 legs and no i didnt have a good way to seal the top and make it look clean so i left it open. The bottom spot on the Unibody has a half circle cut to allow it to drain. I have only been hung up on a few rocks that got stuck inside the pocket. I just may bolt up a sheet of 1/8th to sorta boat side it. But really its not been a problem for me. I dont how ever really beat this thing. I have come down hard from about a foot. And let me tell you if felt solid when it rocked my body. The door to slider gaps are unchanged even with all the bashing its been taking. The slider is only 3/16th so that would prob bend some before the mounts would start.