These are 3 different shelves for you iMac (thin monitor, not the very old thick bezel version).

The Fusion 360 files are also included for your convenience.

This design is an alternative for similar products offered on Amazon, however, my version is less visible and works perfectly fine:

https://www.amazon.com/gp/search/ref=as_li_qf_sp_sr_tl?ie=UTF8&tag=techwithkrame-20&keywords=Monitor Top Shelf&index=aps&camp=1789&creative=9325&linkCode=ur2&linkId=6350ac3bb3699048e11119919b0663ac