The sizes
@Doug71 references are close to where I would be, for a framed door, plus, I would be making this sort of thing for a paying client, so longevity and reliability in service would be my remit, and as such mortice and tenon construction would be my only option.
@Fitzroy's door is an ideal solution, you don't necessarily need a frame, as long as you build it well, as he certainly has.
Picture is easier to show (old one) how, in some cases, I fix my boards to the rails:
View attachment 196420
View attachment 196421
Screws are all in a groove cut in the rails then a beaded strip planted in to finish it off.