James,

I had the same issue with a 17' Sea Tour XL.

Instead of plywood I laminated the upper section of frame 4 (at least it was the front end of the cockpit) in place to make it ~1.5" wide by 0.5" deep. Probably 1/8" plies.
The lamination also raised the shape of the frame next to the gunwale.

There was lots of extra room, I'll make a second post to show the final shape.
All of this was done while re-skinning the boat.
There was more strength in the frame than originally.

Of course the original plywood frame member and the block used to define the contour were cut out to finish.

I didn't expect this level of competence and kindness so fast.

That was great work on the deck beam, when space is tight the difference an inch or two can make a big difference.

I'd love to see how she looked after you skinned her.

Actually with the scarcity of resources for skinning material in the uk how about I just fill the frame up with expanded polystyrene blocks and set sail? ......

Best regards, James