Hi, so actually if you like thinkering you can modify some sram wireless blips which you can usually find for under 100$.
https://www.sram.com/en/sram/models/ec-blip-b1.
This Instagram video shows how you can add some custom buttons (zirbel in this case) to them
https://www.instagram.com/reel/C4EF0wDsmHf/?utm_source=ig_web_copy_linkedit: i did a little more digging and if you have no other shifter connected the blips might not work (sram prevents you from only paring blips with a rear derailleur without any shifter). An alternative is to connect the internals of a eagle axs shifter (
https://www.bike-components.de/en/SRAM/GX-Eagle-AXS-Controller-12-speed-Shifter-p81643/) to your preffered buttons.
There are some videos about that. ([www.youtube.com/watch?v=mfr1iRHcvNM])
I'm curious what you are trying to add (always cool to see people add custom shifters).
Hope this gives some inspiration