Skua

Presumably you downloaded Microsoft Keyboard Layout Creator (MSKLC), which I still had to do to make a custom keyboard layout in 2022. Funnily enough I was also wanting to use AltGr to add diacritics to vowels, because I don’t want to have to go to the backtick key for àèìòù

For what it’s worth, making a custom layout actually is a huge pain in the arse. That’s a pretty niche use case, but there is definitely no reason for it to be as much of a hassle as it is