Gray electrical conduit fittings are straight threaded. A male, and a female and an O-ring will do the trick. Under $1 for the 3/4" size. Home Depot sells some sizes and sells O-rings for the 1/2" and 3/4" sizes, in the plumbing dept.
For a nifty one piece unit with an intake screen, check out the plumbing bin at your local hydroponics shop. They also sell pieces to go larger, at some shops.
For a bit more, and some pictures, use the search feature and look for "conduit bulkhead" without the quote marks.
Here is one site with pictures.