G'Day Paul,

Your question is a bit open - several possible approaches to answer it, or try to...

I often sweat in a smaller ID tube in the forward facing vent, to reduce the effect of ram air as the model comes into, or turns away from dead headwind location on the circle. That is a restrictor....

Others have joined a piece of smaller ID tube ahead of the vent tube opening, holding both with fuel line. To test to see whethere more or less restriction helps, the smaller tube can be crushed in pliers. Easy and inexpensive to replace if "adjusted" too far...

I don't know of other "restrictor" approaches. So long as the open end of the uniflow vent is inboard of and "above" the tank, particularly in regard to the angle of the combined loads of gravity and circular flight, there should be no fuel siphoning.

Is this any help?

The point of a uniflow is to provide uniform flow of fuel to the engine.  I use a one way valve on the uniflow to "protect" the fuel tank from the effects of varying air pressure on the vent.  Also, it is possible for a uniflow to siphon fuel out of the tank.  I've also used a restricted tube.  The hole in my KK one way valves is probably a #80.