DIY hatch on curved surface

I (finally) got a fish finder and wanted to install the transducer inside my kayak, near the rear. Unfortunately, there was no access, so I thought I'd add a hatch. However, there's no flat surface in that vicinity and I couldn't find any good info on installing a hatch on a non-flat surface. So, here's how I mounted a 5" SeaDog deck plate on a slightly curved surface.

First, I went to TAP Plastics and got a couple of scrap pieces of 1/2 inch "starboard", which is a brand of marine grade plastic. That cost me $2. I then cut out 2 rings from the scraps, one for the top and one to reinforce it from the inside.

To shape the top ring, I used a random orbit sander. The maximum curvature was about 3/8", so I had to sand it down to about 1/8" in the thinest spots. This was definitely the trickiest part, but the random orbit sander worked well---it removed material fast enough to make progress, but not too fast to do damage. I worked on it until there were no visible gaps. The only advice I have here is to frequently check your work. You don't want to remove too much material.

A belt sander made short work of the inside ring, which I ground down to about 1/8" all the way around. Next, I cut a hole in the kayak and drilled the mounting holes thru the kayak and both mounting rings.

At this point, I was ready to glue it and screw it (so to speak...). But, there's one technical issue with starboard plastic, namely, nothing sticks to it, unless the starboard is first heated with a propane torch. There are good videos on YouTube showing the process. In any case, you can easily test whether you've heated it enough---if water will no longer bead up on the surface, it's ready for gluing. I was a little nervous about using a torch, since I'd spent a fair bit of time fabricating the upper ring, and, after all, it's just plastic. It turned out to be a piece of cake. I rapidly passed the blue part of the torch flame five times around each ring, then flipped them over and repeated. For the black starboard, it's especially easy, since the heat causes a visible "shadow" effect. After this process, all surfaces passed the water test.

For sealant, I used Life-Calk (an expensive polysulfide caulk, available at West Marine) on the top ring. For the inside ring, I just used silicone. It's bolted down with stainless hardware.

I haven't had it out on the water yet, but I sprayed it all down with a hose, and there's no hint of a leak. I'm confident it's going to hold up.

Nice and clean installation!  Thanks for sharing Jeremy.

Take that people who are scared to drill a couple of holes for mounting stuff.  Looks good.  Solid DIY.

I need to do the same thing to my kayak. Question, what is the large diameter on the hatch?