TOM
As you said Those tabs should be sandwiched between two ribs. I would cut the covering from one tab to the other between those two outside ribs. The tabs are probably separate pieces, meaning there should be a empty slot between the tabs. You can determine this when you cut the covering. I would leave the old tabs in place cause you won't get them out.

I would make a "T" shaped piece to fit in the slot between the old tabs. The bottom of the "T" would be made to fit into the slot between the old tabs. At each end of the "T" would be new tabs that are part of the "T" but extend over the old tab locations. Then epoxy the new piece into the slot between the ribs and to the old tabs.

You may have to have a little of the new piece showing because the new piece is going to need some meat to give the new tabs some strength. Make sure you use a good 5 ply hobby grade plywood to make the new piece.

Good luck

Tom