I mentioned above about the 4" bulkhead fitting I bought from Sacramento Koi. It cost about $68 which was a fraction of the price of bulkhead fittings in general which seemed to range from $108 to $225. I got back the trimmed fitting on Thursday and it looked great.
The fitting is in 3 pieces, the grill, inside and outside pieces. I had the bottom of the inside pieces and the top of the outside piece trimmed to fit within the limited space on my Rubbermaid 150 gallon Stock Tank.
I put the inside piece in place and made sure before I cut the hole that both pieces would fit. Then I traced a circle with a pen to mark where I would need to cut.
After that I removed the pieces, cut the hole with my Dremel jigsaw attachment and then spread silicone sealant on the inside and outside pieces of the bulkhead fitting. Finally I screwed them together along with the grating piece and this is what I got, inside:
And outside:
Note the trimmed bottom part of the inside pieces and the trimmed top of the outside piece.
John