You do not need a clunk on the "fill" line.
After the tank is filled, this line serves no other purpose.
just on the line to the carb.
The way I understand your system, you have a 3-line system.
1-line to fill tank (no clunk needed, unless you use this line to pump fuel out at end of the flying day).
1-line from tank to carb (felt clunk needed here).
1-line used as a vent (no clunk needed).