It took a bit of research and a couple of tools, including a 3/4" x 1/16" aluminum bar, a hacksaw, and a right-angle drill attachment, but this is the finished product, missing just a bead of caulk we'll add to prevent any possible toenail catching if the aluminum were to separate from the tunnel's front wall. I can't do better than Scott D.'s original instructions, which I'll repost here, for easy reference. My only edit would be to pre-drill holes in the aluminum bar piece before doing the bending, as it is easier than doing so afterwards. We used self-drilling #6 x 3/8" pan-head machine screws, which we Dremeled burrs off after installation, as the Martins will have to brush by them every entry/exit. So we didn't need a drill bit. Each guard needs 5.5" of aluminum bar, so you can get 6 of them from a 3 ft. bar or 13 from a 6 ft. bar.
The alternative would have been to buy all new tunnels from a Troyer supplier. About $6/gourd at present prices, but this solution was about $12 of parts plus the $25 drill attachment, which will see future use.You will need some 3/4 aluminum flat bar that is 1/16 of a inch thick. You can use a small piece of 2x4 to aid in making the bends. On the flat bar, measure 1 3/8 and place your first mark, from that mark, measure 2 3/4 and mark, followed by another 1 3/8. Place the flatbar under the 2x4 on the first mark, bend upward all you can as if your making a 90 degree bend. Move to the second mark and repeat. Cut the flatbar at the third mark. Take a crescent wrench and bend both end's until it matches the radius inside the tunnel. You want to leave some gaps from 1/8 to 3/8 of a inch as in the pics. Do not flush mount them to the top of the entrance. You can then attach using a couple screw's or adhesive caulk. Caulk any cracks along the edges. I like to also use my dremel tool and make traction in the bottom of the tunnel. That is about it, once you get the hang of it, they are very simple to make.