The Westerm Field 550 looks like a Mossberg 500, so I would start there looking for a plug. A good gun shop might have one, a gunsmith could make you one, or gunpartscorp.com may have one on the shelf.
It is not difficult at all. It needs 14 gauge wire, freely available any hardware store.
Plain Plug Gauge
A gauge line can leak due to a simple break. It may also be caused from a line tearing away from the plug. The gauge itself may be too large or small for the plug as well.
With a spark plug gap gauge.
to measure spark plug gap
"Gauges" is a slang term for large gauge plug jewelry. Technically "gauge" is the measure of the thickness of any type of body jewelry, including standard earring posts.
A feeler gauge is actually a set of very fine strips of steel used to measure small gaps such as plug gap of an auto plug, or gaps in bearing surfaces.
Use a .5 feeler gauge for the TR Spark Plug Gap.
The spark plug gap is .020 on your gapper gauge!
Depending on which intake manifold you have there may be a plug in it near the thermostat housing. Also, each head will have a plug on the side. One is usually fitted with the sensor for the original gauge. This is the usual spot for wiring in an aftermarket gauge.
Browning.com has manuals you can download for free.
anywhere between 16 and 18 on a spark plug gauge should work fine.