I’m so sorry for my slow response…
Hope I can still help you out by replying :-/
Your mold looks great. The mold I made works the same way and had good experience with it. I only had some difficulties with the metal sheet that started bending a bit after time, which had effect on the plastic sheet that came out. It was not super straight unfortunately. Especially difficult when you want to further work with it in lasercutter for example.
* I didn’t use anything to make unmolding easier. Didn’t had big issues with that to be honest. Sometimes the plastic was melted around the screw-thread or in between the flatbars. But most of the time I was able to remove it after some cooling down.
Sometimes there were some plastic flakes (probably another kind of plastic) that sticked to the mold. Thoroughly cleaning the mold after using with a spatula worked best for me. My experience is that every singel piece of plastic or plastic assessment remaining in the mold, could make it sticky.
Never thought about oven paper. Can imagine it it works fantastic!
* I did a lot of tests with changing the amount of plastic, the temperature and time. On the end I had quite a good idea about the right proportions and settings.
* Holes in the lid could work!
My mold just pushed the plastic out on the sides. But can image that a few holes works better