John, if it helps I attached a couple photos of my latch spring that show the shape Mr. Kettenburg talked about. As he said, having the buttplate hole small enough it doesn't show is the trick.
Since the spring moves vertically in relation to the gun, the hole is a vertical rectangle. The spring is inserted through the hole with the flat attachment point of the spring horizontal. It needs to be rotated 90 degrees so it can be attached to the patchbox wall. That's why that "waist" is filed in at where the flat part stops. You fish it in, then when that waist reaches the hole, it gives clearance to rotate it 90 degrees and then you can insert it the rest of the way.
Nailing it on would require incredible faith in one's ability to know it would work the first time. I must have had it in and out at least a half dozen times tweaking things, so the screws were the only way I could do it.
I saw your PM as I was writing this - I didn't use spring steel, just a nail. For the small amount of bend it seems to work fine. I attached a couple photos of some of that process. I forged the flat part out using a propane torch and the anvil on my vise.
For a first build it's looking great! You'll find this place is invaluable, the folks here are so incredibly helpful!