Pressure Pot/Paint Pot, same thing. You definitely need to use pressure when dealing with clear resins.
Polyester- almost useless in most tin-cured silicones. Polyester relies on heat alone for its cure and will start to cure from the thickest point in the part and the cure then works its way outward to the surface of the part. This means the center of a part will become solid before the surface against the mold. Because polyester has a high rate of shrinkage, the center also shrinks and causes the surface to 'squirm' against the mold, often creating wrinkles or cracks. Heating the mold does help, but if too much heat is used, the mold will swell and become useless (it doesn't 'unswell'). Platinum cured silicones will produce a better polyester part.
Clear Urethanes- most have very long cure times, which helps in that you can wait for some of the bubbles to come to the surface. Only the smallest will remain and either vacuum or pressure can be used to get rid of them. Pressure is better as none of the chemistry will cook-off in the low pressure environment associated with vacuuming. It should be noted that bubbles are not 'shrunk' during the pressure process-- they are forced into solution with the resin.
One thing not addressed in other posts is the quality of the final part. You can get the best casting material made and have it work perfectly, but unless the master is constructed with the intention of creating a 'clear' part, items such as canopies will not be as optically clear as one would hope. The only way to know if a master will produce an optically clear part is if the master is polished such that surface flaws are not observable. In other words, if the master is not clean and shiny like a mirror in the places where the part is supposed to be clear as glass, the 'clearness' of the casting will less than satisfactory.
Given the cost of producing an unsatisfactory part, if this project is really for only one part, you'd probably be better off just scratchbuilding it. Unless you do have a very clean and shiny master.
Scott