Trigger plates are super easy to make as are single triggers.
My first rifle was a Beck like gun, I made the trigger plate and cold forged the trigger.
All you need for a trigger is a piece of cold rolled steel, a vise, hammer and some files and start banging away to form the trigger.
Here are a couple in the forming stage;
Trigger plates are pretty straight forward as well, make them the size you want, cut a slot, bend them to match the curvature of your gun and inlet them.
This one is for an English fowler, a little different with an offset slot as well as one of my cold forged triggers.
In and working fine;
Now, as it being hard to make a single trigger work right, I haven't found this to be the case. Pin it slightly higher than sear with the trigger bar hitting the sear 5/16" from the pin and you will have around a 3# trigger every time. My fowler is 2.8# and my beck is 3.1#.
I am a rank beginner builder, if I can do it, you can to.