Any time you distort vinyl, you're going to lose some fidelity. Let's get that out of the way.
I had stacks of vinyl in the obligatory milk crates. Unfortunately for me, my mom shoved them out of the way when I went to college, and that happened to be a nice blazing hot attic. The records that were tightly packed were fine, but the ones that were a quarter full ended up getting warped. Sometimes you can let it slide, but these were bad enough that it sounded like vocalists were on a trampoline.
I decided to flatten some using an oven. I had a nice flat pizza tin, so I put a record in a sleeve on the tin, then put a large flat-bottomed pot on top with metal handles. I set the oven to 150 degrees Fahrenheit and every ten minutes added a quart of water to the pot. When it was three-quarters full, about two gallons, I turned off the oven off and let everything cool down. It takes quite a while but the records came out flat and sounded fine.
That worked fine for me, but took a lot of time where I had to watch a timer. I got two sheets of Pyrex from a glass shop that was slightly bigger than an album cover and sandwiched the warped vinyl. Now I could get the heat up to 150 and then let it cool down by itself without adding water every ten minutes. Worked out fine over a long winter break and it helped to heat the apartment.
Iron Noder 2017