I will toss in 2 cents. I would not use a 5 gallon plastic bucket in this application no matter how much I agree with you that the pond kits sumps look like over priced plastic.
I also know what some plastic molds cost and it is very high.
Jim did not say it quite like this but the cost of the materials are often irrelivant provided you are pricing the job correctly. Customers can see a 5 gallon pickle bucket and they will doubt your professionalism.
I would agree with you that on a small job, 4" concrete block on compacted soil is a decent foundation for something with less the 20 PSI.