An egg carton is the obvious solution, but I don't have one on hand.
We came up with two quick n' dirty solutions. The first, since it is still nice outside, was to simply draw a board on the porch with sidewalk chalk. James made this one for us.
This worked pretty well, expect that my poor back and feet don't care for the squatting. Ugh. I will suggest that you make your circles generously sized, though, since without any sort of rim there's a lot of opportunity for "splash."
My second brainstorm for inside play was to use Wiki Sticks. I'd acquired a small set several months ago, but they weren't quite the hit I'd imagined and most of them were still untouched on the supply shelf.
I bent a dozen into circles and stuck them on our little Ikea table. I didn't finish off the goal / mancala slots at the end just out of laziness, but I probably would next time.
Using the slightly sticky, slightly raised Wiki Sticks to provide a "rim" was a moderate improvement over a chalk line, but having the whole setup on a table was the biggest win for my 39-year-old back.
Despite Six Whole Years of parenting experience, certain aspects of child development still catch me by surprise. In this case, it's the fact that grabbing a handful of 8+ small pebbles and then dropping them one at a time into little holes (or circles) is surprisingly difficult for a six-year-old. (Of course, this is just a bonus to the game as a learning activity, so I try not to be too impatient!) And sure enough, quite frequently the wiki-stick "cup" came up with the handful of stones. The stickier the wiki-stick the better it's going to work, of course, but I'll admit this is only a quick-n-dirty solution rather than a perfect one.
I have a feeling we'll be coming up with another iteration or two of mancala boards, but that's just part of the fun!