It's a 6" piece of acrylic tubing with cut slots. I simply throw in some bait and lay it down in front of the gobie's burrow. Hopefully after a few days the goby will venture inside. Then while I see him inside the tube I pull on the fishing line that's attached to the top of the tube. Most of the water will drain out, yet leaving about 2" in the bottom to keep the goby wet until I place him in a pail and off to the LFS
There's tons of fish trap designs out there, got this one off the net.
It wouldn't work too well for most fish but for bottom dwellers it should do just fine.
here's the link
http://saltaquarium.about.com/cs/fis...uariumtrap.htm