Ok, so bulkhead fittings are metal? I have a farm supply store here,
(fortunately).. are they also available at Lowe's? I did something similar
with the toilet flanges.. I attached them from the inside, and used the
regualr wax seals for them. I added long bolts, and used LARGE washers on
each side, tighten down,. with silicon between them and the rubbermaid to
add just a little more seal to them. Those look to be working great.

I'll see if I can find the bulkhead fittings tomorrow if possible. If they
are heavy, might that be an issue for the plastic to support?

The ones I use are plastic - I don't think Lowes would carry them as they
are used mainly for industrial applications, not home repair
No they aren't heavy, no extra support required. I used them on a square
Rubbermaid container for a bio-filter for a small pond for a customer (3/4"
inlet and 2" return fitting). The container was a square ~ 28"X20" and 24"
deep - the ones they have in Walmart (blue)
Gale :~)