I don’t currently have one, so would still need to buy one which doesn’t seem cost effective. Plus I have tried that idea, but I’m so weak willed that after a few weeks I gradually began reinstalling stuff (“just for now…”) and ended up with a fully distracting mini-computer next to my bed!
How about you just use an old phone, all apps removed if you like, no sim card in it, no network set up etc.?