I believe that 5 gallon buckets are HDPE (high density polyethylene). That type of plastic is used for things like paint buckets, solvent containers, gas cans, etc, because it is chemically resistant. Most glues aren't going to bond to it. I know TAP makes a special structural glue for polyethylene, but you'd have to order it, and it involves heat "treating" (pretty simple with a propane torch) the plastic first. I have heard of other things that involve sandpapering and heating, I don't know how well they work, and what the local availability is of them. HDPE can be hot air welded (if you have the equipment).