My method of repair for that base would be to weld a piece of aluminium flat bar, preferably 5mm thick x 30/40mm wide depending on what is needed to bring it to the correct skirt length,welded each end and a couple of reasonable tacks across the front where you can get a weld, This is welded on the inside and then you can use body filler on the outside to get the correct shape you need. The aluminium gives it strength from stones and gives the body filler something to hang on to and should be fine for use. The worst thing that could happen is the body filler might crack if you used it too much, so what,big deal fill it again.
