I have made some shims to use with the UCB. There are cases where the decrease in roller pressure as the rollers roll off of the board result in carving depth change. These shims should have been part of the original project. I have added them to my list of changes.
One may or may not need to use them. Once made, however, they are easy to use and one just as well be safe and use them.
I worry about shims sliding off of the rails and so I made them with slots. You need some 1/4" thick material. The slots are 1/2" wide assuming that your carrier MDF is really 1/2" thick. If it is not. you will have to adjust the slot width on the mpc. Do this by dragging the yellow side handle on the long sides of the shims. (See the last picture.) The left side for the left two, the right side for the right two. Try and get the right number in the width box (rail width - 0.125). (Typing the number in the box does not properly scale the width, you must drag.) Some hand tuning or a few tries may be required. (Only 8 minutes to cut.) It is nice to have a set with a snug fit.
The four shims should not be used to set the board height. They may not all be exactly the same thickness. I think one should use a single shim at all four corners to set the rail heights.
Use is simple. After the board is measured, slide on the shims just before you clamp down the head.
(The front shims are different than the rear shims because of the stop blocks in the front of the carrier. Removing these stop blocks is on my list of changes and so the shims will be all the same for the improved UCB.)