Unfortunately, hunt long enough and at one point or another, you'll end up learning about using other capes on your own mounts.

Yes, I learned the hard way on a beautiful double-white throat patch buck that had especially sentimental value as my wife helped me locate it in a very tall bean field carrying my four-month-old son in one of those pappoose-style carriers as we walked grid searches together. She was actually the one that found the pin-prick of blood that pointed us in the right direction to where he lay.

I stupidly took too long to get him to my taxidermist and his cape was slipping by the time I got him there.

In your case, your taxi is probably going to have to buy a cape and he'll just measure it to determine the size form he needs to order; no biggie at all as far as that aspect goes. Make sure he has seen the rack to know what general size to order (you don't want to put a really big rack on a smallish-sized deer, or vice-versa).

Good luck.