You can reprogram one of those cheap Chinese STM8S based products https://github.com/TG9541/stm8ef like:
$2.5 free shipping thermostat https://github.com/TG9541/stm8ef/wiki/Board-W1209 big pcb makes this kinda ackward to mount
~$4 W1209WK dual 3 digit display https://github.com/TG9541/stm8ef/wiki/Board-W1209WK you could reprogram this to display cpu temperature on one lcd and MHz on the other 😀
~$2 my absolute favorite, voltage meter https://hackaday.io/project/16097-eforth-for- … other-voltmeter hard to buy the right one, most $1-2 voltage meter modules on ebay use dedicated measuring chip nowadays, but there are still STM8 based ones available, for example https://www.ebay.com/itm/174081875242
of course this requires some programming and soldering skills, or motivation to learn
Edit: Just realized you already have case with a specific hole 😀 forget the above.
I looked at https://github.com/wiretap-retro/2-digit-Turbo-Display and while it is using ridiculously cheap and easy to program ATtiny85 its also complicated because of it, not enough pins means its also using two SN74HC595DR shift registers so cant replicate using off the shelf dev board, need this exact pcb manufactured.
Instead https://microdigisoft.com/2-digit-7-segment-d … h-arduino-nano/ https://www.instructables.com/Arduino-Nano-LED-Display/ Arduino Nano is $2, dual 7-segment display will also be $1-2, you need to measure your hole to get the appropriate size one.