Uhm, this was a quick hack, to have something to use in a few hours.
I will work a bit more on the ui, but that's it.

If nobody beats me to it, I may do a port of free42 next... that one has a two lines display, but I suppose showing the four stack values can be arranged.

The excalibur you cite seems to be "freeware", not open source, so it is not an option for me.

Also, I have an hp32s at home, which is not too different from an hp42... and rpn9 which seems based on hp42 as well, and I'd like to have something like that.

I hope that helps!