Here is a little Backpack Holder that you can print and install on your desk. Mine is 28mm Thick but you can modify the file directly in Bambu Studio to fit you desk. 

I used the default 0.28mm Extra Draft print profile with 15% infill and it work great.

I hope it will be good for you, let me know if there is any problem.

Here is a step by step tutorial to modify the .stl file directly in Bambu Studio :

First step :

• Select the part you want to modify and then select the measurement tool :

Seconde step :

• Select the plan 1 (Blue) and then the plan 2 (purple) :

   

  

  

Last step :

• Click on “edit to scale” and replace the current value with the thickness of your desk.