I use the "lost foam" method.

Carve/sand a block of pink or blue foam to the shape and size you need.

Cover the foam with a low temp film (Econokote, Towerkote) to insure the foam will not melt from the resin. (Some resins WILL eat/melt the foam)

I then either apply PVC mold release or put on a minimum of 10 coats of wax. (provided you want to save [or try to save] the plugs).

Cut your fiberglass cloth into strips about 2" wide and about 6" - 8" long. The weight of the fiberglass will depend on what you are using it for and the size of the part. For a 40 size cowl I normally use either 2 layers of 2 oz cloth or 1 layer of 4oz cloth with a layer of 2 oz over it. Again, this will depend on the overall size and shape.

LIGHTLY spray several strips with 3M #77 and apply them to the plug. You should have at least 1/2" of overlap on the edges/seams. Do this to all the strips before you apply them so they will adhere. You will want a minimum of 2 layers, again, depending on the size/shape of the part. I have found that 2 layers works well for 40 - 60 size cowls. It provides the rigidity and strength I want/need without being overly heavy.

I use laminating resin for mine. Mix up about 2 oz of resin (again, depending on the size of the part) for a 40 - 60 size cowl. You want to mix it so you have a 30 - 40 minute WORKING TIME. Now dilute this 10% BY VOLUME with acetone.

I use plumber's flux brushes to apply the resin. Apply the resin and be sure the cloth is COMPLETELY saturated. (if you see white spots, that area is not saturated) Once the cloth is saturated, CAREFULLY squeegee off any excess resin with a playing card (if you do not take the excess of now, you will be sanding it off later) . Let this dry for about 48 hours. NEATNESS COUNTS IN THIS STEP.

Once it is dry, you can look the part over and decide if you need to add another coat of resin to fill the weave or not. If you need to fill the weave, do it now with the resin. Do not dilute this batch.

When dry, sand the part as needed. I use spot putty to fill any low spots (comes in a tube in the automotive section and is red in color ).

When the part is sanded and filled, be sure you have nothing hanging over the back of the plug.

If you are going to try to reuse the plug, take the part and immerse it in water for about 30 - 45 minutes while GENTLY flexing the fiberglass part to allow the water to get between the fiberglass and the plug. The water will allow you to "pop" the part off the plug.

If you are not interested in saving the plug OR the part will not pop off (the RC Gods like to play jokes on us mortals ), take the part outside and make an X in the back of the covering. In the X pour a little gasoline (2 - 3 oz is plenty for a 40 - 60 size cowl ). Come back in 20 minutes and clean out the sludge and dispose of properly.

Thoroughly wash the part inside and out with soap and water. The part will be very soft and flexible. After the part is washed, do any final reshaping with your fingers and set the part aside. It will need about 72 hours to harden up.

It sounds hard, but it really isn't - just messy.