Tucker, If you are going to make molds you first have to deciedeif they will be male or female. The majority of the time in this hobby they will be male. If you are using an existing part to make an additional copy you have to be sure you can protect it. In your case the kit is no longer available so I woulsd be very cautious. Plaster will work if you use it to make a male mold. Since plaster generates heat as it cures, I would fill the mold partially and put rocks or something in the palster to take up space and also cut down how much plaster you need to fill canopy. Less plaster less heat. Vaseline will work as a release agent. I would try all this and something a little less critical. Get some packaging material. You know how everything comes in those clear plastic packages that you have to cut open? Finally if you get a few bubbles you can always fill it with some bondo or plaster. after all these are only temporary molds.