I started out with an enclosure made of ABS sewer pipe, and then moved up to a customized PVC design. Here's a picture of Rev. 2 of that, which I constructed for a client.
The top is a ring sawed from a 3" Schedule 40 white PVC cap. Those caps are domed, so I cut the ring and then glued in a top sawed from 3" white drain pipe cap to make a flat top. The flat top is necessary for the bracket detailed below.
The side of the ring is tapped for a 1/4-20 nylon hex head set screw so the top can be twisted to align it to north. There's a shallow saw kerf across the diameter of the top to aid in aiming north.
The antenna is mounted with nylon screws and nuts to the underside of the top with a gusseted bracket made from a sawed down plastic project box.
The body is 3" clear PVC Schedule 40 pipe. There's no particular reason for clear except that I like seeing the antenna inside (and it makes the picture more interesting). White would work as well.
The base is another PVC ring cut from a fitting, probably a coupling. That's glued to the body tube and to a cut-down PVC reducing bushing.
Then there's another reducing bushing to get the ID size down to match the mast OD.
The mast is an old fiberglass two-way radio antenna about 4' long from which I stripped the guts and connector, leaving just the tube and metal mounting section. It's more rigid than PVC pipe.
The CAT5 cable runs from the antenna down through the mast and out the bottom.