I had the exact same issue on my XL, though only a 1/4" gap in the same places (back left/front right). I don’t think you’re shimming in the correct spots. You need to shim the X rail in the places indicated (back of the left end; front of the right end). I used a stack of 3 slivers of aluminum from a Coke can in each spot and it closed the gap exactly. If you look closely, you can see the shim in the second picture. Coke can wall thickness is ~ .004" thick, so with 3 layers my total shim was .012". In your case, you might need 4 or 5 layers. Give it a try (and get rid of those washer shims on your Y’s)
