Doug Fleenor makes a stand alone box that is custom. It is pricey, about a grand if I remember correctly, but it is rock solid I am sure. Another thing you could do is get rs-232 to rs 422 or rs 485 converters as serial is only rated for 50 feet and 422/485 are rated for much longer like a few thousand feet. And you could probably just use a DMX line for it since it is shielded computer cable essentially and does 485 normally. That would be far less expensive. Maybe $30 per adapter and you just put one at each end unless the projector did 422 or 485.

Let me know if you want more specifics on it.

the link to the DFD page: http://www.dfd.com/232.html