If you were to use the same math to create the same type rangefinder as used by WWII "Dam Buster's", only in wire, with the far, spread wires ending in two small posts, between which your picture area will be found,
your make shift tool would allow you to have an idea of what you're pointing it at.
A cross piece to hold true the far ends cam be soldered or epoxied, and if you want, to a flat slide in plate so the front of the finder does no need to extend beyond the lens, where it could be seen on the picture.
This is just a simple gig, in case someone wants to try it, even though, there are many here that raise concerns against doing so.
In fact, if I want a too expensive item, I wait until I can afford it, even if it takes years, but I suppose there are others that would want to get an idea of where they are pointing their camera.
Cheers.