Supposedly the shape is that of a coonskin nailed to the wall:
Dave Nelson
You're in luck! Kemtron designed the proper headlight bracket complete with coonskin numberboard years ago. It's still in production as Precision Scale Co part #HO-31607 (brass), or part# HO-31608 (plastic). You may be able to adopt the stock headlight or replace with the lost wax headlight designed for the bracket.
Hope this helps,
JB
You could just buy one from Bachmann. They're not currently offered, but they were in the original production run, coonskin, doghouse and all. There are two currently on Ebay (in rough shape, but the price is great!)
Of course, "Friscoizing" a Russian Dec into a SLSF engine will take a little more than just adding the correct number board. The Bachmann Frisco engine is a good start as it has the correct tender, doghouse and number board, but there's more to it than that. Looking at surviving SLSF Decs, it appears that they were each a little different, having different headlight positions, walkways, cabs, and even appliance placement. So if you want to model a specific engine, you'll have to start digging up photos of them all and find one that you like to concentrate on. Then it's just a matter of hanging brass details on the model!
Ray Breyer
Modeling the NKP's Peoria Division, circa 1943
Alex