You did a FANTASTIC job restoring your nagauta! - either, get Kyle’s book “Shamisen of Japan” - you can get it here, or on Amazon, or look in the “construction” part of the forum right here on Bachido.
Let me see if I can link MY thread when I made my doukake - it’s super easy, just takes a lot of drying time …
Okay, here is my thread from when I made mine … It kind of starts in the middle of the process …
Saran wrap your dou - Basically, take your shamisen apart, leaving just the dou. Wrap the dou in Saran Wrap. I mean, REALLY wrap it - going both ways - you don’t want to get it wet!
Basically, making a doukake is paper mache … covered with cloth
Make your paper doukake form - get Brawny paper towels, or some other thick cloth-like paper towel - the other ones are too thin - and soak one sheet at a time with Mod Podge. (In his book, Kyle says to use Elmer’s but I found that too thick. If you DO use Elmer’s, thin it out with some water.) Start layering and smoothing the glue-soaked paper towel over where the doukake would go. Smooth it as you go, making sure not to rip the paper, and smooth out any “lumpiness” as you go. Put Saran Wrap over THAT, and let it “set” overnight. In the morning, take off the Saran Wrap, and let it completely finish drying.
Now you have your “form” - it’s really big! You’ll have to measure the sides and cut down - measure them a quarter inch bigger, because you might have difficulty cutting the form, and you want leeway to even it out after cutting the form so that it’s “doukake-sized”. After that, sand it so that it’s nice and smooth - I didn’t do this and mine came out a little lumpy.
Now you have a nice shell! I think you can see the rest in the thread - let me know if you have any questions! 