I know this is kinda weird and some people might not get it but i made the meowl meme but reverse (instead of an owl with the head of a cat is a cat with the head of an owl). i call it mat :)
At that 400$ I’d spend 200 more dollars and buy a ROG ally X or a steam deck (heck I could buy one of those for cheaper than that) or I’d just wait till they make a cheaper revision
For either you would only need one controller but if you use a GameCube controller your going to need to buy a second Z button and if u use a classic controller your going to have to buy GameCube buttons but they sell them in numerous different colors on aliexpress and eBay for less than 7$
For case design seeming how you want to make it similar to a controller for a phone I’d say try something similar to shock slayer’s wave rider or ik I mention this console a lot but the Nvidia shield portable is basically an android phone with a controller on it so you could make a design...
Me personally i’d see if I could give the person a reasonable amount of money to either get the ps2 portable or get the retroid mini back and then not go to the person to buy things after