As Steve mentioned, PVC tubing is easy to purchase at your local home improvement store and easy to work with.
A really easy approach is to take a 4" PVC pipe coupling, cut a short piece of 4" pipe and glue it in one 1/2 of the pipe coupler. Then either on a lathe or a belt sander sand the end that you glued up as square as you can get it. What you end up with is a .5" thick end cap that you can drill and tap 10-24 machine screws for the end pressure bulkhead....... next do this for the other end.
I use a silicone based rubber for the gasket. It is actually rubber stamp sheeting used to make rubber stamps in a laser engraver..... bonus if you have a laser engraver to cut the gasket out...
This rubber is very flexible and makes a great seal.... It was bought at Laserbits.com....... http://www.laserbits.com/index.php?main_page=product_info&cPath=80_316&products_id=880
As you can see in the two pictures, I constructed both ends and then coupled them together with a piece of pipe cut to the length I needed..... (The rubber gasket.....is the lighter gray between the end bulkhead and the main tube....kinda hard to see in the photo)TIP:
Prior to screwing on the end bulkhead, flat sand the ends of the bulkhead with fine abrasive paper on a flat plate such as a piece of glass of flat piece of marble.... This will make sure the ends are perfectly flat and will mate with the gasket for a waterproof seal.
And yes the 1.5" long 10-24 cap bolts hold really well in the PVC pipe. If you should strip one out, a 10/24 heli-coil works great. (They have enough hold to crush the rubber gasket)