Advice on part making idea

For a small part like that, I'd skip the oven and heat the foil with a tea candle.  You'll have more control and can add a bit more plastic if you need to to fill any airspaces that form at the bottom of the mold.  Make sure you have more shaved plastic than you think you'll need, about double the amount...   It's trial and error with this process, so don't give up on it after a couple tries...

I dunno how big the part is, but I would think taking an appropriately-sized piece of solid plastic stock and sanding to shape would be easier (and probably safer) than making a mold and fooling with molten plastic. It's amazing what you can create with sanding sticks and a little patience - I made a 1/144 IJN torpedo out of a piece of sprue of the right diameter, trimmed to length and then just sanded to get the rounded nose on one and and the tapered rear on the other end:

I've been thermo-forming and vac-forming my own parts for a long time mfsob, since I was about 13 or so and I can promise you that it's easier and faster than sanding by leaps and bounds.. Sure, you gotta be careful or you may get a burnt finger now and then, but the chances of suffering any injury needing more medical attention than the bathroom medicine cabinet can handle is extremely low...  You run a greater risk of injury using a hobby knife, soldering iron, or Dremel tool than you do thermo-forming...  So unless you have a propensity towards stirring molten styrene with your fingertip, don't sweat it... It's no more "dangerous" and only slightly more difficult than stretching sprue for radio antennas...  Besides, if it's turned into a liquid thin enough to spill, you can bet that it's also on fire...