I don't think there is a tortoise that could fit in with a box turtle.
A redfoot or yellowfoot might have a compatible environment and diet, but both get too large for easy apartment living. A redfoot might be prone to carrying diseases, and unless your box turtle is captive bred, it too may carry diseases.
Also a box turtle would probably like it a little cooler than a redfoot.
Hingeback torts might be compatible, although they too probably prefer it warmer than a box turtle would like. And aside from Bell's hingebacks, I think it's hard to get them captive bred and therefore healthier. The wildcaught hingebacks seem to be unhealthy with parasites and other problems.
A russian, greek or hermann's tortoise would be small enough, but they all like it warmer and much drier than a box turtle. Also a possible cross-contamination problem.
I think it's too much hassle with a high potential for heartbreak to mix a tort with a box turtle.
Just get another box turtle of the same species. Or even better, let the one you have live by itself. They don't really need company, and aside from breeding may view other turtles as competitors and therefore as sources of stress.