Is the trolley the heating element? Would a front and rear interference plate to mate the trolley to the sheet holder reduce incidental air flow for more uniform and faster heating with a more contained air space between the sheet and the heater element? Perhaps the addition of an internal low speed circulation fan to homogenize the heated air volume?
I don't know anything about vacuum forming really. I've done some DIY vacuum forming with a pegboard screwed to an MDF box and a hole drilled in the side for my shop vac. LOL. My results were mixed from passable to "Holy crap! That looks bad."
My first thought about your project was the position of the trolley handle. I am not crazy about reaching across the plastic sheet when its heated. I just have this fear of shedding workshop dust from my protective gear onto the hot sheet, and not being able to remove it when the project is complete. Maybe a pull rope and a pulley to slide the trolley back without any personal proximity to the heater elements or the heated plastic?