A tiller extension swivel allows one to sit on the rail of one's sailboat, yet still steer. Tiller extensions are also referred to as 'hiking sticks'. A swivel can be purchased for minimal cost, but I like to build things, so...

Step 1: Materials and Tools

Materials:
Small piece of PVC pipe. I think I used 2 inch diameter, about 3 inches worth, leftover from a plumbing project.
QTY 2 #8x3/4 flat head wood screws
QTY 1 #8x1 flat head wood screw
QTY 1 3/16 x 3/4 "binding post" or pin with retaining ring
Spray glue
Pattern - attached later
Scraps of wood

Tools that I used, suggestions as to what one could use will follow in the steps where the tools were used:
Heat Gun
Clamps
Bandsaw
Drill press
Stationary disc sander
Screwdriver
Sharp Knife
11/64 drill bit
3/16 drill bit
13/64 drill bit
Countersink for #8 flat head screws

Step 2: Flatten the PVC Pipe

Cut a section of PVC pipe to rough length, about 3 inches. Split the pipe along its length into two half circles. I used my bandsaw for the cutting, a handsaw works just as well.

Heat one piece at a time until it becomes soft and rubbery. Do this outside, I'm guessing it releases some fumes that you don't really want to breath. I used a heat gun (think a hair dryer on steriods), one could heat in boiling water or an oven. I'd avoid using an open flame, though placing it on a pan in a hot grill might work. Wikipedia says the melting point of PVC is between 100º and 260º C (212º-500ºF) I've tried the boiling water (100ºC) method and I don't think that is quite hot enough. Anyway, temp isn't important, soft and rubbery is. Experiment.

Once it is soft and rubbery, flatten out between two scraps of wood and clamp. High clamping pressure isn't really necessary, just enough to hold it flat. Allow it to cool. Repeat with the other half circle.

Step 3: Pattern

I drafted the pattern using CAD software. One could free-hand it, but I've found it quite easy to make nice looking stuff using this method. The pattern should be attached here as a .bmp file. Open it with paint, print it out and check the distance between the dashed lines on the piece labeled "B". They should be 5/8 inch apart. Check the width (top to bottom on the page) of the pieces. They should be .800 inch (a little over 3/4 inch, a little under 13/16 inch) wide. If so, move on. If not, see if you can play with the printing scale (file, page setup) and get them to print the right size. The easy way is to measure what printed out and work out the scale. For instance, if it prints out with a 1/2 spacing instead of 5/8, divide 5/8 by 1/2 and multiply by 100. That equals 125, so change the scale to 125% and print again. It should now measure 5/8. Coincidentally, if you want a wider or narrower extension, play with the scale to get it to print how you want it.

Cut around the perimeter of the A and B pieces of the pattern. Don't cut on the lines, leave a margin of about 1/8 inch or so. I used my sharp knife, use scissors if you like. Spray one of your pieces of PVC lightly with the spray glue. Stick a pattern to it. If you used 2" PVC pipe like I did, both patterns should fit on one piece. Print out another pattern and make two or toss the other piece in your scrap materials drawer/box for future use.

Step 4: Cut Out Pieces

Now, cut around your patterns. Don't bother trying to cut on the line, just get close and stay on the outside. I used my bandsaw again, handsaws will work too.

Refine the shape by sanding to the lines. I did this with my stationary disc sander, sandpaper on a block of wood will work. If you're going with the 'by hand' method, it might pay to cut as close to the lines as you can. Perhaps use a file or rasp to get close before going to the sandpaper.

Deburr using a sharp knife. Don't cut with the knife, but rather scrape. Hold it so the blade is perpendicular to the piece and scrape.

Step 5: Drill Some Holes

Hole sizes are marked on the patterns.

For pattern A, drill the outside holes 11/64 inch and countersink for #8 flat head screws on one side. Drill the center hole 11/64 and do not countersink.

For pattern B, drill only the center hole. The outside holes will be drilled together at one time after bending. Drill the center hole 3/16 inch and countersink for #8 flat head screw.

I used my drill press to drill and countersink. A hand drill will work too.

Step 6: Bend the Ears

Bend the pattern B piece into a flat bottomed U shape. For this step, you will need something 5/8 inch wide. I cut a scrap of 2X2 lumber to 5/8 inch.

Clamp the 5/8 inch piece of wood on top of the pattern B piece, aligned with the dashed lines of the pattern. Clamp to the edge of a table or workbench with one "ear" overhanging as shown in the picture.

Heat the overhanging ear using the heat gun. It will likely try to curl and return to a tube shape just a bit, that's ok. Get it nice and rubbery, then using a scrap of wood, fold it up against the 5/8 inch thick piece and clamp in place. Allow to cool, then un-clamp and repeat with the other ear. I shot a video of me bending the second ear:

There's that heat gun again. This step would be a little more difficult without it. One could probably clamp the workpiece using all metal clamps to small pieces of wood and use the other heating methods, but it would be difficult.

Step 7: Drill the Remaining Holes

Cut out pattern C. Cut on the lines for this pattern, it is only guiding the drilling of the hole. Spray the outside of one of the ears with glue, line up and affix pattern C to it. Drill the hole(s) in the ears. Use a 13/64 inch drill bit and no countersink. In the pictures, the wood block has been removed. Leave it in place and drill right through it.

You're getting close. Heat the pieces gently one more time. Not so hot as to get rubbery, but hot enough to soften the spray glue. The patterns should peel off easily. Allow to cool and clean up any remaining glue with acetone on a rag (outside again). Hang the rag up outside to dry. Wadded up and thrown in a garbage can is inviting spontaneous combustion and a fire.

Step 8: Assembly

Screw the pattern A piece, the base plate, onto your tiller using the #8 x 3/4 flat head screws through the outside holes. Screw the pattern B piece, the swivel, down on top of the base plate using the #8 x 1 flat head screw. Don't over-tighten this screw. You want the swivel in good contact with the base plate, but not so tight as to prevent it from turning. Attach the tiller extension to the swivel using the binding post (shown) or a pin with a retaining ring.

Go Sailing!