Well, you can break down the autotile and by doing so only make the rim. That ought to do what you want it to - like this:
It's a pain to find the right tiles in the expansion, but by just making the rim it ought to do what you want it to.
By the way, to expand the autotile you just double click it.
And of course, another option is to make the rest of the land the opposite undulation. and leave the part where you can walk alone.
EDIT::
Also, if you're worried about lag, just get Zeriab's anti-lag from the database