I just found out about Uniseals and they have some appeal for joining
a radiused surface with a tube. The hole is cut with a hole saw,
which helps to keep the hole round, or to the shape of what is going
into the hole. I haven't tried them yet, but I have a couple 4"
waiting to be put into service.

http://www.aquaticeco.com/index.cfm/...etail/iid/8600

I have always wondered why everyone here complains about the prices of
Bulkheads, until now - I followed the link above to them and couldn't
believe a 3" bulkhead (equivalent to what I use) listed at $70. At a farm
equipment dealer & supply depot in Canada, that bulkhead is $12. They use
them on the big plastic tanks that hold hundreds to thousands of gallons, to
haul water, liquid fertilizer and weed chemicals. Is it the fact that it is
listed for 'recreational pond use' ?
If they were that expensive, I would also be looking at Uniseals
Gale :~)