The TTRPG-9000 is an open-hardware and open-source handheld electronic dice rolling computer for table top RPGs. It allows the user to select NdM to roll and shows the result of each individual roll as well as the total. A psuedo random number generator is seeded/updated via both the clock and user interactions with the board making for good randomness in the output. It runs off of 2 1.5V CR2302 batteries and has a very good battery life (tested to 40+ hours and going).
If you want a unit, but do not have the expertise to fab/solder the PCB, program the software and 3D print the case you can buy on Etsy from my store.