I have side rails as suggested by (Atlas), I have a different option if you dont have the special parts for step 2, and I also have a mouse pad holder that will hold a full sized mouse pad!
Check out my original stand here: https://www.instructables.com/id/Knex-Laptop-Stand-and-Cooling-Pad/
I am currently working on how to make the stand adjustable and yet sturdy so it wont smash over.
let me no if yoiu have anything i could add!
Step 1: Side Rails
Pic 2: side rails attached.
Step 2: Alternative to Original Step 2
Pic 1: Alternative piece, make 4
Pic 2: Original
Pic 3: What it looks like now
Step 3: Mouse Pad
pic 1: what you are making
pic 2-4: make this with the different angles
pic 5-6: make this piece
pic 7: line up
pic 8: attach
pic 9-10: make this piece
pic 11-12: make the other top piece
pic 13: line up
pic 14: attach
pic 15: line up top and bottom
pic 16: attach and done