RE: Making a glass cowl from the inside of a plastic one ?
Ditto on Speedy's comments!
I have also done this for years. It is much easier than having to make a female mold, first . . . saving a lot of extra time and effort. The loss in thickness is negligible. Just take care of it with a quality wax and PVA as described several times now, and you should be just fine. Realize that as a "mold", the plastic is not nearly as durable as a thick fiberglass mold. It is thinner and more brittle, so it won't last forever. But you should get several copies before you start having issues with the plastic cracking, splitting, etc. If you only need a couple copies, this is a fairly ideal method, all things considered.
But, If you later find this is a favorite plane, and you're making a lot more, save one of your more perfect lay-ups as a new master. Then you always have something you can use to make a female mold later, if needed.