Looks a little long. Stamp it, punch it, put your loops, grommets, dye it, wet mold last if needed. For the straight grip stock you may need a strap around the back to keep it from slipping forward.
When the lockdowns started I bought enough plain rice and enough dried beans to fill a 5 gallon bucket each. Used up most of the beans, that rice won’t get soft if you boil it all day.
The Colt Frontier 44 mag in the Al Stohlman book might be close. I have no idea how to create a digital pattern or I would send you one for a New Model Blackhawk.
I think he said that he could not have either gun in shop. If he wet forms it it might stretch enough anyway. Or just trace 1/2 or so outside the smaller pattern. There outa be a new model Blackhawk pattern floating somewhere. You can make it where the adjustable sights sit above the top of the holster.
Another thought. If the same customer wants both holsters, make the Colt holster first and let the customer see if it also fits his Blackhawk. If so, make 2. If not, make a little bigger one.
The New Model Blackhawk will just be a little beefier in frame and cylinder than a SAA. Big adjustable sights vs a groove sight on SAA. Probably 1/2 inch all around extra. Old Model Blackhawk very similar in size to SAA. Colt Army is the Colt Single Action Army, Model P, Model 1873.
Will Gormley patterns. You can order direct from him or Weaver Leather. I know he has several for Colt Army and I imagine some of them could be modified for a Blackhawk if he does not have one. Main difference is large adjustable sights on Blackhawk. New Model is a bit thicker than Colt Army, old model Blackhawk and Vaquero.
Here is a machine model vs. bobbin chart
https://www.thethreadexchange.com/miva/merchant.mvc?Screen=CTGY&Category_Code=sewing-machine-bobbin-style-info#Consew