Duplexer for portable VHF repeater
You've got a lot of projects tied up in this project!  What is the actual spec?  If it's only to make a solar powered repeater, consider loosening some of the other specs.  Designing and building a circulator would seem to be good project of its own.  The big problem seems to be keeping the transmitter out of the receiver.   So you might have to construct a better front end for the receiver.  Lowering the output power will directly help the problem.  So will using a split wider than 600kHz.  And some repeaters work well using separate transmit and receive antennas. Using vertical antennas, and locating the receive antenna above or below the transmitting antenna (where the radiated energy is lowest) will help as well.