Tighten rope without turnbuckles?
Use a simple cinch loop: 


Connect to the first eye and tie a small loop about 2/3 the way across the span. 
Run the tail through the second eye and back through the loop. 
Pull the tail toward the second eye to create the desired tension and pinch the tail to maintain position. 
Tie off the tail at the loop. 




Source

The key here is the pulley effect, which compounds the user's strength, and the friction of the assembly, which helps hold position while tying.