The RG351P/MP is I think about as good as such a device can get but it scales up extremely poorly, especially on an x86 where you'll probably use the sticks more.

I used to have something called the JXD S192K and it solved the issue easily by having the controller keep a traditional shape independently from the screen. In particular the triggers were very recessed compared to the top of the screen, where they would have been too far away.

Of course that made the whole thing bulky and not something you would carry without a case.

I really like how the Deck handles it, by having the input follow a thumb moving diagonally, in order to keep a mostly rectangular device.

This video is how you do not do a rectangular device.