What if you turn the crack into a hatch. Cut out a rectangle maybe 6"x12" around the crack, get a new piece of plastic 10"x16" x 1/4" thick and glue/screw the old piece to the center of the new one. Buy some spikey T-nuts or glue nuts to the back of the new piece of plastic. Put the patch inside the hole so the original plastic is flush with the rest of the drum, bolt the hatch in place with taper head bolts so they are flush (after drilling the appropriate holes). If your never going to remove it, you can silly cone the overlapping flange to seal the patch. Also you should try to curve the patch to match the drum so you don't have a flat spot, a heat gun should work. A piece of poly 55 gal drum may make a good patch as well. Good luck Maxwell
Bill