Square buckets are ideal.
I tried Uni-Seals for my build and they failed me miserably. They did not leak from installation, but several split at the factory seam during my testing phase. People swear by them and they work for some people, as evidenced by the many pictures of them in action.
I took the whole thing apart and did it with bulkheads. If you get decent bulkheads they will suck the bucket wall flat when you tighten them. The good ones have nice thick washers. Leaks are unacceptable and bulkheads assured me that.
You can even get double threaded bulkheads so that you can screw filters, screens ect to the inside, and a Male PVC adapter to the outside.
Just so you know I am not talking shit.
Look closely and you can see that the other one is splitting as well.