Heating plexiglass, keep protective film on?

Hi, I'm trying to make a visor out of 3mm plexiglass sheets. I don't have a vacuum table so I made a plaster plug, placing the sheet over it and putting it in the oven at 300F, hoping gravity will be enough to make it sag down to the right shape.

Should i remove the protective film? I tried heating a sheet without it and it stuck a bit on the baking tray giving it some minor imperfections. I'd like to keep the protective sheet on the fiberglass to keep it in perfect condition but would anyone not recommended this? I want to make sure before I catch the house on fire or seal the plexiglass permanently between the two protective sheets.

I don't think its a good idea putting it in the oven! What you need to do is find a way to focus the heat only where you need it modified. Like some sort of heating element, or a good heat gun. Then get it just hot enough to get the job done. Oh, and you do want to remove the protective coating. It will make it easier to focus on the area your working on, and also not burn it onto the plastic.
That's my thoughts!

I tried doing that method with ABS (only a little hotter) and I can't say it worked to well. The plastic will sag over the mold, but it requires pressure to seal it firmly against it. I resorted to donning some oven mitts and pushing the plastic in with my fist. I ended up with a slightly wrinkled piece of plastic that vaguely resembled my mold when all was said an done.

get some plastic called PETG , it is clear and made for vacforming, plexiglass gets bubbles in it and stuff. on tint. dye it with liquid cloths dye, make sure the plastic is onthe thick side so it will not curl up when you dye it. the water and dye needs to be just aout boiling. dip it in and take it out, allow the viser to cool then do it again and repeat until you get the tint shade desired.....................

I'm not sure what I have is plexiglass or PETG, I got it at home depot, it was marked as acrylic sheets

You need to remove any coatings from the plastic weather bending or blow/vac forming.

If it says acrylic on it then that is what you have.

If the shape is simple enough then I would suggest making the form out of bent sheet metal.

Also to heat the acrylic it's best to bake it on sheet metal then wearing heavy gloves move it to the mold.

I spent 7 years making acrylic aquariums most having bent edges and even blow forming bubble tanks like the one in Picards Ready Room on ST TNG.

In high school in Industrial Design I made some sun glasses by heating the cut shape then forming it over a curved piece of sheet metal.

PETG is good for vac forming also thin sheets of polycarbonate can be used.
For something like that you want your buck to be made of solid acrylic or Ultra-cal stone.