The following assumes that you have the type of portlight that opens by hinging upward at the top and swinging inward. If not, ignore...
One question you want to decide is whether you want to be able to close the curtains when the ports are open (up position). If so, one solution would be to put cup hooks at the upper, outer corners, of each port, run a piece of shock cord between the two hooks, and stitch the curtain material with a tunnel/tube to run the shock cord through. When the port is closed, the shock cord runs along the top edge of the port. To open the port, unhook one end of the shock cord, pull the cord/fabric to one side, open the port, and rehook the cord running below the opened port.
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