I have to connect a syringe to a squared glass microcapillary. I looked in many online shops but I've never found a proper fitting thus having always liquid leakage between the microcapillary and the connection.
We often make specific tube connections by carving PMMA plates using carving machine instead of commercial connections that are often not convenient for practical use of rectangular capillary. Or could you use soft tubing to connect them?
There is an epoxy putty which you can mix and shape and then harden in an oven. That worked well for any fluid and gas tight connection, however a permanent solution.