Hey there!
Assuming you have a little hardware know how, the best way would be to bolt on a bridging piece. Glue and silicone both work, but can let go, and that could get nasty if you have a full tank. If you are unsure what/how, read on:
Get a (minimum)1/16" thick strip of polyethylene or polypropylene plastic about 3/4" wide, and long enough to bridge the broken part with an overlap of 1" on each side. Drill a 1/4" hole 1/2" from each end of the plastic. Place the plastic over the broken bridge, mark where the holes land on the bridge, and then drill holes in the bridge. Use 1/4" Stainless steel bolts with washers on each side and nylock nuts (nuts with a nylon insert) and fasten snugly (not too tightl!).
Don't know where to get plastic? find a clean white commercial pail, rinse thoroughly and cut out two horizontal strips from the side. Why 2? because the pail is probably thinner than 1/16".
Obviously, if you are not a handy kind of person, you may need help with this method.
Good Luck!