Honestly, you will HAVE to handle some original German guns to even begin to get things close to right. I see so many modern "Jaeger" rifles that are stomach-wrenchingly wrong. Even if well-executed.
Being in Nicaragua, no doubt you're not likely to run into any 18th century German rifles, though... And you can't get mail in Nicaragua? How do you get your barrel and parts???
German rifles come in many forms. Some of the forms are kind of hard to pinpoint exact locations on, while others are unmistakeable. A rifle from Wien will look nothing like a rifle from Berlin, which will look nothing like a rifle from Frankfurt am Main.
As far as general dimensions, well that can change from here to there too, but think buttplates of around 5" tall, some more, some less, and 2 1/4" or more wide. No more than 2 1/2" of drop at the heel (generally rather less), fairly level combs, REAL nice flat cheekpieces... actual cheekpieces, not horrible, hollowed out, jaw-jabbing chin pieces.
1/4"-3/8" of cast off at the heel, toe out, cast off at the nose of the comb, wrist 1 1/2" high and wide (minimum, for most normal guns. 1 5/8" is often better)...