It may be robbery, but when you consider the fit and finish, trial and error getting the mold shapes right, how you're going to attach and waterproof the thing, then the layup and getting the gelcoat just right, cuz you just know that homemade is subject to a much, much higher level of scrutiny, it'll be time for a nap, and when you add up all the costs, handy or not, do you want to spend the rest of this (and maybe next) winter (or this one if the freakin thing never ends) or just buy the damm box, mount it and get on with enjoying that new plotter.