Screw the rat tail file, no offense Lou.

OK Joe, here we go . . .

First off, you need to locate the center of your spinner with respect to your prop. You cannot just place it over it and eyeball it (trust me). If your spinner has a center screw, go get a long screw, or a dowel to hold your spinner centered over the prop. If your spinner doesn't have a center screw, then come up from the bottom of the prop so that the spinner centers itself on the inner cone. Carefully lower the rod until the spinner touches the prop.

Hold it there.

With you spinner centered over the prop, use a pen to place a mark on the spinner where the leading edge of the prop just touches it. Make a mark on the prop blades also, don't worry; you should be able to wipe off the marks later. Do this for each blade.

Measure the height of your prop (put the prop on a table and measure up from the table).

Make a vertical/perpendicular cut into your spinner at these marks to the depth of your measurement. A razer-saw works best.

Again place the spinner over the prop using the reference marks; this time the leading edge should line up with the vertical cuts you have just made. Now view the spinner from the front, with the prop behind it ( it will be a birds-eye view if its on a table), approximate where the trailing edge of the prop will be.

Visualize the cross-section be studying the shape of your propeller blades. It should resemble an arc.

From the point of your deepest cut to the reference mark of your trailing edge. Draw the arc on the inside of your plastic spinner.

Keeping safety in mind, remove the material the best way you know how (I usually use a dremmel to remove the bulk of the material and then finish it off with a sharp xacto blade). Follow the shape you've drawn inside you spinner and and remove small amounts of material to avoid slipping the knife. Keep test fitting the prop throughout the process. Allow a slight clearance for the fitted prop; the prop should not touch the spinner.

My spinners fit like a glove over the prop, but keep in mind if you increase the pitch of the prop, you have some more widdling to do!