Boy are you lucky I am still watching this thread!
Ah, Annapolis, I spent the longest four years of my life there- yup, I was educated at that "trade school on the shores of the Severn River" where everybody wears the same clothes.

Of course the training I received and working for my "Uncle" for 22+ years allowed me to retire young and have time to sail a Hobie.
To cut it to length. Take piece of brown paper or poster board (even newspaper will work) and wrap it around the tube. Once the edge overlaps and is aligned over your mark, trace the edge with a pencil and it will give you a perfect line around the circumference. I used a jig saw with a very fine toothed blade to cut it, but a circular saw will work- just be sure to raise the blade. Files and a disc or belt sander may be needed to clean up the edges.
Cutting it lengthwise- First you need to mark where you want to cut. Brace the tube on a flat surface and lay a long, straight (1 X 4) board next to it to serve as a guide to help you draw a line the full length of and parallel to the axis of the tube. Then measure half way around each end from your line to mark the ends of and draw the other cut line- if you want it cut exactly in half. I think if I make another one, I will make it with higher sides and a smaller top. I think I used a jig saw to (carefully) make these two cuts, but again, a circular saw will work also. The key to a straight line is to use a guide or fence for the saw. I used a 10' length of 1 X 4 which I clamped to each end of the tube and used that to guide the saw.
The lip is 1/4" thick PVC trim or lattice(?) strip that I got at Lowes. It is attached with PVC cement and screws.
The end pieces were made from ply, which was cheaper than PVC or other similar materials. If you use ply, use exterior grade and seal it well. I used urethane glue and screws to attach the ends.
I may eventually put a small dehumidifier (available on Ebay) in it because moisture condenses on the inside at night. So they don't mildew, I currently remove sails, PFDs, towels, etc. from the tube when I get home. Since I have occasionally backed the trailer in the water too far and partially filled the tube, I drilled a few drain holes in the aft end of the bottom half of the tube. I angled them aft so water on the road wouldn't splash up into the tube. I may also add some sort of open weave (narrow strips of Dri-deck?) plastic/vinyl flooring in the tube so nothing rests on the bottom of the tube. Additionally, I may add ram air vent holes somehow so that rain or road water can't get in them.
I couldn't find cheap SS piano hinge so I used nickel plated hinge I got at Lowes, and painted it. I used SS wood screws to attach the hinge. Be carefull that your screw tips don't come through on the inside of the tube and if they do grind the tips off- you don't want to rip your sails, PFD, hands, etc. PVC taps easily so you can also use SS machine screws to attach the hinges and other fittings. I used some nice SS spring catches I had around my shop to keep the tube closed. I will replace the cheap utility handles, they bend too easily from the weight of the tube.
I haven't made a new fiberglass "warhead" to replace the one I turned from MDF. It is is too heavy and the MDF absorbs moisture, so I will eventually replace it.
Best of luck.