Hourglass would be my first thought -- one such that, as soon as the sands ran out, the summoning's duration ended.
However, a sieve would also be cool -- something to "purify" the sand that passed through it. Basically, it'd be like a big Britta filter, but for sand -- many layers of mesh, getting finer and finer -- you dump a load of sand in the top, shake it for one full round (or however long it takes), and where the purified sand strikes the desert sand, you get your elemental.
Or a maraca. (Made from the bone of a SANDGORGON!)
-- N