The only part I basically remixed here is the LCD spacer. You will need M3 bolts and M3 threaded inserts. 

Threaded inserts are used for

• the back plane.
• the LCD spacer from both ends. Use the original LCD spacer if you don't want to use inserts there.

The particle sensor slots right into the backplane (see photo).

The license for my own designs here is Public Domain. Do whatever you want with it :)

Onshape link for the whole design.