Uniform depth Cut that follows complex surface. Text on plastic tank

I have a project where I design a plastic tank that needs to show the volume increments on the side.  The shape of the tank is pretty complex and nothing is really flat.  I need to sketch and then cut the volume marks and volume size in the side.  Below is what I'm working on.  The back was flat and I have increments of 25, 20, and 15 gal shown.   I need to do the same on the left side all the way down to 5.

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