I made a ps2 style joystick out of a broken ps2 controller (I only had the printed circuit with the two mini-joysticks) and a wii empty shell.
It plugs directly to the C= joystick port (no adapter). Since the PS2 joy is analog (two potentiometers) I used a couple of diodes to keep the axes separated.
Yes, I know, it's ugly. It's awful. I'm the worst 3D designer in the world (as you can see from the 3D printed monitor too) and I had no patience to make a nicer job, actually LOL.
But it works fine, at least with ZetaWing and Neutron. Will test with Briley as soon as I will receive the Kung Fu Flash cart.