I cobbled together a quick-n-ugly to hold 2 sit-on-tops and my plastic CD Storm. Just got some cheap 2x4 & 1x4, and some metal straps.

Basically construct 2 tall "L" shapes, maybe 6' tall by 3' wide (bolts, or glue & screw boards together), tie them together with boards forming an X between the uprights (in a plane perpendicular to the feet of the L, so the 2 L's stand side by side about 4 - 8 feet apart). Add another horizontal bar about 3 feet above the floor (making them sort of an inverted "F" shape), brace with the metal straps so the structure will take some weight. Add some pipe insulation on the tops of the horizontal pieces for padding. Make sure either the padding or the horizontal bars (or both) have a slight upward angle, which will tilt the boat toward the upright pieces.

As long as it's standing against a wall, it's stable. You can put two kayaks on edge on the bottom, one (or two) on edge on the top.

Ain't pretty, & might not stand up to a 6.5 earthquake without being bolted to the wall, but it's quick & cheap & does the job. Might have to expand it slightly (or add retaining straps) when I start storing my wooden boat on it.

There are also commercial rack systems available - look in the back of a paddling magazine & you'll find some ads. And without a whole lot more expense & effort than I put in, you could have a very pretty & robust rack. But it's possible to do functional for under $15 material & a half-hour labor.