jason,
Uniseals work really swell. They don't leak (in a non-pressurized system) so no sealant is required. To get the pipe to slide through the seal easily, spray the pipe with a window cleaner. Might even be able to use liquid dish-washing detergent and water.
I do foresee one possible drawback, though. If the wall thickness of the filter vessel is thin and somewhat pliable, the side of the vessel might warp near the hole you cut for the uniseal and that could possibly lead to a problem.
To save some money, you could also use PVC electrical conduit fittings instead of bulkhead fittings. I did this with some of my round HDPE barrels. I used a rubber O-ring on the inside and applied a layer of 3M 5200 marine sealant on both sides and screwed the male and female fittings together. Worked great at a cost of about $6 for both fittings compared to $25 or so for a bulkhead fitting. Of course the O-ring and sealant were an added expense.
Gordy