I looked at that board for a year and tried to figure out how to do it. Mine weren't too bad but one was chewed up a little and the end was broke off. I almost thought I would use them over but really wanted to make them new. I even asked some cabinetmaker types as well. No one could tell me how to do it and I assume it was done on a band saw with a special jig that would be difficult to duplicate.
I finally figured a way to make it and I am happy with the results.
With the originals removed I measured the maximum dimensions and I believe it was 1+1/2 by 1+1/2 by whatever length. I cut that out of a piece of 8/4 white oak.
I then measured off station marks every 6 inches starting at the front of the old board and transferred these station marks to my new board. As it appeared the original board had a 90 degree corner with the outside face to bottom face making the 90 I felt I could assume my new board should be the same. I then measured the lower dimension and upper dimension off the old at each station mark and transferred these dimensions to the new.
Then using a Japanese pull saw I cut through each station mark until I got closing to each measurement, basically defining what was to be removed from the new at each station mark. Yes it does change angle every lineal foot of travel and by cutting into the new boards the right distance at each station mark you have established the face of the inside of that board.
I then used a bandsaw for hogging off the bulk of the material that had to be removed and then finished with a power plane and finally a hand plane to the cut lines and finally a faired face that created the inside face of this piece. With that I checked the fit up to the hull before steambending and made a few adjustments where necessary. I then placed it in the steambox which only covered the thickest 8' of board. With help I quickly bent it in place tight up against the hull and clamped with C-clamps to hold for a few days so it would set. Not until I was able to secure with screws did I get the tight fit I wanted. However I did have a few areas where there was a little bit of gapping but adjusted with a block plane and filled the rest. These filler boards are by far the most challenging piece of wood to recreate on the Thompson. I would guess I put about 5 hours apiece into making them.

Rod Syverson