I was at a Maker Faire and saw the M5Stack Cardputer and thought it would be the ideal platform to build a RPN calculator on, such as the HP 10C. After using Microsoft Word to make an overlay and coding in Arduino, I was able to make a working model. There are some limitations, such as the small size, and the firmware is at a beta stage (for a complete list of features/issues see the Github webpage), but it's easy to update the firmware using a standard USB-C cable and a Windows PC.

It is fast - it can do 100,000 iterations of a loop in about 10 seconds. And cheaper than what is available on auction websites for between $150 to $250. I added some features, such as 2 font sizes, 3 font color options and audible keyboard beep option. Also, 99 program steps are available with all 10 registers.

Please note that the picture with the overlay stickers installed is for reference only. You will receive an overlay sticker that you cut out and place yourself. There is also a beta version of a 32LC and an alpha version of a 42LC on Github that can also be downloaded to the M5Stack Cardputer hardware (but you will need to print your own overlay sticker).