So I made a run at filling the unwanted bow plane limber holes and so far, so good. Here's the process I used:
I traced the hole to be filled onto some graph paper held to the part from the back side.
Then I cut out the tracing to make a template.
Next I traced the template on the plastic stock sheet of the same wall thickness as the bow plane area.
Using some small metal snips I cut the traced part out of the sheet and then worked it to shape using a sanding block and coarse sand paper.
Here the plug part is compared to the hole. Looking pretty good.
Once the part is properly fit to the hole, I taped it from the front to keep it flush with the outboard contour and applied Pro-Weld from the back side to glue it into place. With the part secured, I took off the tape and glued it from the front side as well. Any small gaps were filled with baking soda and hit with CA.
Overall, a fairly simple fix. We'll see about the row of small round holes on the aft end of the part. I was hoping to create a punch out of brass rod to stamp the plugs out but I'm not sure the size of the hole will be a standard tubing i.d.