I made my own grid spot from a rubber plumbing coupling, a bunch of drinking straws and an elastometric fastener.
It cost around $3 all up! The straws are about 12cm long and makes a spot about 30-40cm at a distance of 1m. Here are the pics.
It's a tight fit on the SB900 head (with the filter holder also) - nice and easy on the SB600 (but with some rear leakage).