I know of a college in Edmonton where they used to plant a garden on the inside of the hockey rink every summer. There was no problem whatsoever. They grew absolutely massive carrots and lots of potatoes. They did nothing special by way of preparation -- they just tilled it and planted it every spring.
In the fall, they mowed whatever remained of the garden and took it back to bare ground. Then as soon as it was consistently cold
enough, they would flood the rink and get it ready for skating.
No outdoor rink is ever completely flat; there are high spots and low spots. Flooding becomes a self-leveling process. It really helps if you get 6 inches of snow on top of the frozen ground --- that speeds the flooding process significantly. But if you notice that some areas of the garden are unusually high, you could just take a rake out there and level it a bit, albeit, you'd want to do so before the soil freezes. If it were me, I'd just pay attention as I gardened and would do my best to level things out throughout the
gardening season. By October or so, when everything is cleared and the frost has killed whatever had been growing, your soil
should be flat and ready to go. One last trip over it with the mower and you're ready to flood.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf