I'm no expert on making anything of decent quality
But I have rigged quite a few germans for nothing but the cost of hay.
What I did was I lent my really good german martingale out to a friend, ended up needing it, so I rigged up some leather and bailing twine. The redneck fix as my trainer has been calling it.
I took two trands of twine, tied them together around the ring on my cinch (A makeshift clip; just a little knot I could undo later) and then used a little band to hold the together about mid chast before the two individual piece of twine seperated to thread through the cheek pieces. From then, I just threaded it and tied each end to my reins herever I needed them.
I will tell you it's nothing pretty but it works. Also probably not what you are looking to make, but if you ever need a quick german martingale then that's what I would suggest.