Select a Light
For the kayak light project, we are going to use a Shoreline kayak stern light. These lights are available almost anywhere.

Shoreline stern light

The Shoreline kayak stern light is available at retail and online stores for less than $20 USD. We got ours on eBay for about $16.
Get Mast Parts
The light is going to be mounted on a piece of PVC pipe. We are going to use four feet of 1/2" SCH 40 PVC pipe.

PVC pipe for light mast

You also need to get a 1/2" PVC cap and a 1/4-20 stainless steel lock nut. All these things should be less than $5.
Disassemble the Light
Unscrew the bottom part of the Shoreline light. Before we started taking it apart, we put some batteries in it to make sure it was working.

Unscrew light base

Use a hex head wrench and remove the gray part of the light body from the black plastic base.

Disassemble light base

The gray plastic light piece, rubber 'O' ring and hex bolt will be used to make the mast light.
Drill PVC Cap
Take the 1/2" PVC cap and use sandpaper to make a flat spot on the top of the cap. You don't have to do this, but it makes everything a lot easier.

Sand a flat spot on the PVC cap

Drill a hole in the center of the PVC cap so the 1/4-20 bolt will go through it. 5/16" is a common drill bit size that would work.

Drill hole in PVC cap

Once the hole is drilled, it's time to assemble the mast light.
Attach Light Base to PVC Cap
The next step is to attach the gray light base to the PVC cap.

Fit light base to PVC cap

The hex bolt goes through the light base, then the 'O' ring and into the PVC cap using the 1/4-20 stainless steel lock nut.

Use silicone sealant between the light base and PVC cap

Before bolting the light base to the PVC cap, use some silicone sealant between them for extra strength. You don't have to do this, but it makes it a little stronger by taking some of the pressure off the gray plastic light base.

Use JB Weld for a stronger bond between the base and cap

For a stronger bond between the light base and PVC cap, use JB Weld adhesive or waterproof epoxy.
Attach PVC Cap to the Mast
Once the light base is attached to the PVC cap, all that is left is to attach the PVC cap to the 1/2" PVC pipe. The PVC tubing we have is about four feet long. That seems to be a good height where the light is easily visible by other watercraft and the mast is not to "whippy". You can use PVC cement to attach the cap to the pipe, but it will be on there for good. If you use silicone sealant, the cap will stay on and you can probably remove the cap if you had to.

Attach PVC cap to the mast with silicone sealant

Slide the PVC cap on the end of the 1/2" PVC, make sure it's on there for keeps and wipe away the excess sealant with a paper towel.

Attach PVC cap to the pole

Let the silicone sealant cure overnight and the kayak mast light is ready to go.
Using the Mast Light
You can stick the light mast into any fishing rod holder. If the fit is too loose, use 1/2" ID pipe insulation for a tighter fit. Of course you can fabricate a mounting spot that would fit your kayak configuration.

Kayak mast light in fishing rod holder

Rechargeable 700mAh AAA batteries will keep the light on for at least six to eight hours. People tell me they can see the light from a quarter mile away.

Kayak mast light

Always wear a PFD, especially in the dark. Even with a light, be alert and aware of what's going on around you. Safety is always the top priority.