gameboy mode (rule)

using an android phone running retroarch, an 8bitdo Zero Two wireless controller, and this 3D printable mount https://www.thingiverse.com/thing:5998094 .

works pretty good for gb/gbc titles, and its much smaller than other controller attachments! I've been using it for some gba stuff too, like Game and Watch Gallery 4 or Pokemon: Too Many Types, which don't require the L/R buttons.

if this makes you want to buy a controller, I recommend the 8bitdo Micro, which is the same size but with two extra buttons.