What keyboard do you use? There is already an AltGr key on most keyboards.
If a keyboard physically has two keys labelled ‘Alt’ and two labelled ‘AltGr’, then at the protocol level both ‘Alt’s are (indistinguishably) USB LeftAlt and both ‘AltGr’s are USB RightAlt.
(Mac keyboards' ‘Option’ is the label for USB Alt and acts like Windows' AltGr.)
In fact I swap my Alt and Control keys, and don't use AltGr for its original intent at all, because using AltGr+7/8/9/0 on the German keyboard to type {[]} is horrible. I use an english layout (which I modified with my AltGr+j/k/l/i thing and a bunch of other tweaks) instead.