I did an LDC royermod with a 10:1 beyerdynamic mic input trafo backwards.
Worked very well.
It has a case (mumetal) but it is small enough to fit in most LDC mic bodies. Or if you are like CJ, you might be able to "uncase" it.
They can be had for pretty reasonable money.
Of course Jensen sells a 12:1 step down that was spec'd in the original royer article ( DB/E-U, the -U, means uncased (I did one that way too, and it works great)).