I've taken 1 piece of metal and bent it to 30 degrees up and then 30 degrees to level again and then put a 90 degree lip on the end. I took 1 flat piece of metal, and riveted it to the bent metal to make a kind of 'mouth'. It works great, and it's about the only thing that is available for dealing with this issue, other than finding used serviceable stuff around.
I've drawn it out, and if I remember correctly I used countersunk rivets, but drew it with universal head, so you have an idea of what i'm talking about, ( I drew the side profile.)
If you need to go around corners you can pie cut it, and rivet it in the shape that you need, or you can hand form the bends around the corners, with a little practice, brute force, and ignorance.
Hopefully the drawing helps

I think I stole the idea off the internet, trying to solve the same problem as you, on a customers plane, years ago, and it works.