Instead of relying on the weather (we have a lot of overcast days here), I put warped plastics between two Pyrex cake pans in the oven at 170 degrees (which is as low as my oven goes), and check it every 5 or 10 minutes until they’ve flattened out.

Read this thread. I did this and my results are posted in response #29. Don't rely on the sun.

https://pinside.com/pinball/forum/topic/how-to-safely-flatten-warped-playfield-plastic#post-4717547

I'm sure the 170 was relative to the Farenheit scale.
I've used this method and increased the oven temp as high as 215-220 F for one very stubborn plastic, but only left in the oven for less than 5 min and checked it frequently. 170-200F is typically a safe range as long as you don't lose track and leave in too long.

I have the same issue with my Big Chief cab. The Mylar on the back is pulling up too. Wonder if there is a way to fab a new one since the wheel is key to game play

I made one back in the day from the CD/DVD sized clear plastic found inside a new stack of CDs/DVDs.

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I'm working on making my own now. I found a 4" plexi circle on amazon (https://www.amazon.com/gp/product/B071D6QBZP/ref=ppx_yo_dt_b_asin_title_o02_s04) and some laser printable water transfer decals.

tktlwyr You wouldn't happen to still have that graphic for the wheel, would you?

No. Sorry.

In the event anyone else with a Big Chief *snicker* runs into this issue, here is the solution that worked for me.
1) Purchase a premade 4" diameter 1/8" thick plexiglass disc (here is one from amazon https://www.amazon.com/gp/product/B071D6QBZP/ref=ppx_yo_dt_b_asin_title_o02_s04) It's slightly smaller than the original, but it still fills the space below the surrounding plastic completely. It's also slightly thicker so if you decide to use incadescent bulbs rather than cooler LEDs, it hopefully won't warp again.
2) Purchase laser printable water transfer paper with the opaque white background (again, amazon https://www.amazon.com/gp/product/B07WVWB4M5/ref=ppx_yo_dt_b_asin_title_o02_s04)
3) If you still have your original selector wheel, use it as a template to mark and drill holes in the new piece of Plexi
4) Once the holes are drilled peel the top protective layer off the plexi disk. Using the image attached below, make a 4" X 4" print onto the water transfer paper (I made a scan of my old wheel and cleaned it up as best I could while keeping the original look. You may want to lighten the colors, but it matches my machine well)
5) Use and exacto knife to cut out the small hole and larger hole from the water transfer paper. This will make it easier to line up. If your decal doesn't line up correctly, it will display the wrong wheel section in the game.
6) Follow the instructions for your water transfer paper. For mine, it needed to soak for 30-60 seconds before transferring.Slide it onto your disc lining up with the holes, and use a damp towel to gently work all the water out from under the transfer and smooth wrinkles. Let dry for 3 hours.
7) Once it is dry, give it a spray with some clear gloss spray paint.

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