A machine to do this could be costly ($$$$) since you'd be cutting 3.5' by 2' and probably 1' ? What would you cut the mold from? What ever it might be expect a lot of handwork to finish the mold to a sufficiently decent finish afterwards. The cheapest way to machine it would probably be to build a machine to cut that worksize.
A traditional alternative is to make up the part (pattern) by hand in foam. Then if it's a one off use the foam shape as a male mold and lay up your fiberglass directly. If it's for more than one, take a mold off the pattern and get out the filler polish wax and pva.
Fiberglass Coatings inc on 28th st. in St. Petes is a good place for this stuff by the way, and they sell small quantities..