You can add a thick airfoil to your Squirrel with just one piece of balsa that is the same as the leading and trailing edge.

Here’s an old document that shoes an easy way of doing it.

In a nutshell you run a spar from winglet to winglet. You attach it to the front of the winglet so it braces the winglet. You cover the top and bottom of the wing with tissue.

You can notch it to go over the wing handle or remove the wing handle. It could be installed before the wing handle and then the wing handle sliced so one piece is ahead of the spar and the other is behind the spar.

Advantages:

-It strengthens the winglets by bracing them.

-It reduces wing warp.

Disadvantages:

-It’s more work.

-Nobody has proven the extra steps are worth it.