If you decide to go with the foam, I usually cover mine with scraps of LOW TEMP film. Depending on the foam and the resin used, the resin can eat the foam.

If you want to try to save the plug for the future, put about 10 - 12 coats of wax on the film covering the plug (seriously ) and/or use PVA release liquid.

I usually do any sanding of the cowl/part prior to removing it from the plug. When you remove the part from the plug it will be VERY flexible. Do any minor shaping at that time, then put it aside and let it sit for 3 or 4 days - it will harden right up.