<The Feno wing is quite small and light (only 3'-4" , without wing tips) so I figured I'd keep the B/C mount as light as possible .>
Rob,
The bellcrank mount is not the area where you want to save weight. It is the heart of it all and it needs to be strong enough not to get ripped out when the plane hits the end of the lines after cutting across the circle with slack lines. You don't have to overkill it either but think on it before you put in a mount meant for a 1/2 A. What you have is probably ok, I would go with the 1/8 in. Just make sure that both the top and bottom mounting plates are attached to the spars with epoxy. I would also consider moving the plate up to be flush with the top of the ribs if possible so that the center sheeting can be glued to the mounting plate also. Just moving the plate up so it can be attached to the top sheeting adds strength without adding weight.

Picture below shows a top and bottom plate of 1/16 ply which is attached to the spar for about 8 inches and will also be attached to the sheeting when the center section is covered. This works well for 40 size planes. As you can see I notched the two center ribs so the mounting plate can be flush with the top of the spar and easily covered with the sheeting.

Rob: The way Greg shows would be your best bet. Use 1/8" ply and I wouldn't mess with the lightening holes, you can always cut out some weight someplace else.

Hi Steve ,

The Tom Morris B/C's are axially mounted , between an upper and lower ply mounts .
I haven't finalized the mounts yet and the (my) photos are still a mock-up .

Rob Killick