I made these corners for my Lowrider v2 CNC. They sit on my 1/2" thick mdf waste board but the key is that the mdf sheet is 97" x 49" which is one inch larger on each dimension that normal 4' x 8' sheet stock.
With all four corners in place I can drop a full sheet of 4'x8' material in and it will be perfectly centered.
There are three different heights for the corner files:
Tall sticks up 3/4" above the 1/2" waste board
Medium sticks up 1/2" above the 1/2" waste board
Short sticks up 1/4" above the 1/2" waste board
The photo is of the tall version but I'm going to give the short version a try next because it is hard to locate the origin of the router bit in corner if your cutting material thinner than 3/4"