Unless you get heel-clearance issues, there is no reason - except maybe aesthetic - that the rack has to be perfectly level.
Many shops will have stays lying around.
Even
stays (I am pretty sure this is what you mean by 'extenders') that do not meet your exact needs can bring the rack closer to level without too much difficulty. Pre-bent stays can be nicely flattened out if squeezed in a bench vise, and straight stays can be bent to fit. And short stays can be bolted together to make one longer stay.