define custom axes in .designspace file to generate a variable font through fontmake?

I’ve been trying to implement a variable rendition of Wim Crouwel’s New Alphabet typeface by following this guide. I did get to generate a working .tff with a standard weight variation, but would like to customize the axes so they work in tandem to how Crouwel conceived the New Alphabet system.

Is there a way to include custom axes inside the .designspace file in a way fontmake understands? Been trying to do so on my own to no avail.

.designspace file here.