When we were just running the Snoop Express cover a flat box under it looked fine, but with a switcher body we wanted to be able to see the wheels some and have a more correct sub frame profile,
After laying out the lines I cut the 45 degree angles with a Jig Saw and a very good blade that would not twist or bend.
Next I set up the table saw for the long cut at 1 1/2 inches from the top.
I then slowly raised the blade up into the wood while holding it securely to the fence.
The corners were finished with a flat saw as a round blade would cut too deep on one side.
Even with no sanding it all looks smooth and straight.
Now you can see the wheels go round and round!