Any tank can be a uniflow tank. Simply hook the vent line up inside the tank like another fuel pick up line without the clunk. Tie the 2 fuel lines together with a piece of loose fitting shrink tube so they stay together as they move around the tank.

If your plane has the fuel tank on centerline with the spray bar or nearly so you can test to see if this will work before doing it. Fly your airplane inverted in a race track pattern. A normal tank inverted is a uniflow. Your engine should be slightly leaner inverted. If while inverted your mid range rich problem goes away the uniflow tank will work upright or inverted.