I also upscaled the bolt 5% using Cura (so now it's 105% original size), and now it fits exactly (a bit tight!). Else it was too loose.
I haven't tried to mount mount it yet, but it looks nice. I hope I didn't cut my belt too short!
The bolts maybe are too long (if you need to adjust all that length, you have a big problem with your printer!), and the posts also look too long, no need for that.
Anyway great design, thanks a lot!