To the right is the front view of my STR fuel shutoff, plunger on top right is for  overflow, model is fueled through uniflow vent, plunger on the top left it the shutoff reset. Unit stands off the fus by 7/8" to keep the fuel line and fuel flowing outward to the engine, I run a Nelson with the bakplate reversed and the venturi pointed to the top of the head. Plungers are initiated with your left thumb pressing downward while refueling. I open the fuel first then push the overflow, this helps get a prime into the venturi.
   Triggering shutoff is done via .008" cable (red arrow) attached to a .025" trigger wire,  that's seated in a .025" milled groove (blue arrow) on the plunger body and, it's wired onto the body (green arrows). The groove is about one half the trigger wire length. This arrangement gives it it's tension and if more tension is needed, you can add more wraps of wire and, this can easly be done at the field.
  Below is a photo of my BTR fuelshutoff. This one has some screw in studs so it can be dismantled when removing from the model, threads are 8-36. When assembling, just bend the wire over the end of the body, cut/grind so the wire doesn't hit the thread and then solder. Red arrows point to holes in plunger and body, these are for inserting a wire to hold it while screwing and unscrewing the extender.
  The most time consumming part of these assemblies, besides machining, is to get the tension just right. I make my springs on a drill and mandrel with different wire sizes and pitchs til, I get it.  Sizes are about .008 for 1/2A,.012 or, .014 wire for the bigger stuff.  
  I realize that most of these assemblies require some machining abilities but, I just want to show you the ideas I've come up with. I design the units and try to keep it simple as possible and logical, when activating.
  On the right as an inline fuel filter for those cramped type models. I made a die to form a fine screen to a shape and size, that I can push into a piece of 1/8" surgical tubing. It's design is more for catching those big chunks from a new tank more than the little ones, if it can though that screen then, it should pass by the needle valve assembly.