I just realized I haven't posted updates as I said I would. Unfortunately, I didn't have the forethought to take pictures of the process. My woodworking skills have historically been so lacking that I didn't think my process would be very exemplary. But, as it turns out, I am not too displeased with the results, so far. Here is the first panel loosely in place to check for fit:
The caulk on other two panels to complete the transom is curing right now. I'll sand tomorrow, and nothing left to do by add hinges and stuff. Not perfect if you look too closely, but an upgrade.
Even though I didn't take pictures, I'll describe what I've done, in case anyone is considering taking this on.
The panels are constructed like this:
There is a 1/2 piece of plywood that makes most of the body of the panel. It is surrounded by bullnose trim milled from 3/4 inch thick stock. The bullnose is cut so there is a 1/2 inch deep notch on the bottom to accommodate the plywood. This leaves the bullnose top to raise 1/4" above the plywood. 1/4" strips are laid up in this space on top of the plywood.
I bought some 2x4 ipe and some 1x4. (The 1x4 resulted in quite a bit of waste, but I'll use it for something.) I also got a sheet of 1/2 inch sanded marine plywood.
With the 2x4, I set my table saw to rip 1/4 slices, resulting in strips with a thickness of 1/4 inch and a width of 1.5 inch. I used a Diablo thin-kerf blade which cut like butter, did not burn the wood, and allowed me to get a little more out of a 2x4.
Milling the bullnose was kind of pain, but mostly due to my inexperience. I milled it on the router table, using a 3/4" half-round bit for the rounded edge, and milling the 1/2" notch with a few passes for a straight bit. This process might have been pretty easy but I had to relearn some lessons about how to use a router table. Ipe is really hard, so you need to take your time.
With everything milled, the only tricky part is getting the corners cut perfectly to make a nice miter join. It is hard enough to get a perfect 45-degree angle, but two of my panels had odd angles. I have a very basic miter saw with really coarse angle indication. So, I cut about 1/8 inch long at my best approximation of the angle (within a degree or so). Then, I finessed the angle through trial-and-error on a belt sander.
With the frame laid out, I measured the interior polygon where the plywood would go, and cut plywood to match. Again, I cut a little generous and refined to an exact fit on the belt sander (my absolute favorite tool). When I was pleased with the fit, I taped the corners tightly with masking tape and temporarily clamped the trim to the plywood. Then, I drilled some slightly countersunk pilot holes through the trim and about 1/4" into the plywood.
I sealed the plywood with epoxy formulated for that purpose. For the first panel, I did not have the forethought to do this before gluing everything together. So, I had some issues with the very thin epoxy bleeding into the ipe trim. I was able to sand this out, but I learned that it was way better to seal the plywood before gluing it together. That's how I did the next two panels.
I glued the whole thing together with 5200 and screwed it in place. When cured, I removed the screws and drilled about 1/4" deep holes for plugs. I cut some plugs on the drill press out of scrap ipe. Note, I learned in the first panel not to get adhesive near any seams because it will squish out and become really difficult to remove, resulting in an unsightly white line at that seam.
I 3D printed some little spacers that I used to maintain equal spacing between the 1/4" slats. After cutting to fit and doing a dry layup, I laid them up with 5200, using the spacers to keep it neat. I learned that the best way to do this was to do the dry layup and use this to mark where all of the seams will be. Then, I laided down a single bead of 5200 just to the side of that line. So, when I laid a new slat in, I put it so that the edge just covered the bead, then slid it all the way over into place. (I know that isn't a good explanation, but the bottom line is I "smeared" them into place to get an approximately even distribution across the bottom of the slat without squishing much adhesive up into the seam.)
With that cured, I caulked with a generous bead, pressed firmly down into the cracks. I did not mask the seams with tape and am glad I didn't. It would have been unnecessary work. While the caulk cured, I epoxied the wood plugs into the holes. I finally learned that the smart thing to do is to draw a line on the top of the plug, indicating the direction of the grain. So, when I inserted the plugs, I could easily align the grain with the surrounding wood.
Let everything cure for a couple of days, then knocked the tops of the plugs off with a sharp chisel and shaved the raised caulk off with a razor. Then ... sanding ... lots and lots of sanding.
Finally, I cut some notches out for hinges and such and resealed those areas with epoxy.
