You can definitely build a case to Intellijel specs, and they’ve published those on their site if you need em.
I built a 7U case, shown here with a 1U row in the center and a built in Intellijel PSU to avoid needing a power module.
I’d recommend looking into prefabbed side rails to bolt your module rails into if you can find appropriate ones. If you are building a case and manually measuring/drilling everything, it is difficult to get the rail spacing exactly right. This was my second case and I laser cut the pieces, which meant they were precise…my first needed some filing and such to get things to work.