ReRouting Ac Register

My grow room is located in my garage. I have a room that I built that has worked great until it got too hot. As the temps in my garage can inch up over 100 degrees, I will need to do something to cool this area. I was thinking I might split one of my ac ducts and run a 2 to 4" line into the room as a register. The room is 1.5'x3'x5' with a 600watt HPS but that has it's own ventilation via a vortex fan. The room is insulated. If I kept ventilation to a minimal and only blow in cool air when my a/c unit comes on, which is a lot where I live, then would this keep the room cool enough u think?

I'd probably change the AC duct and see how it goes. Keep in mind that your home AC duct system is "balanced" for not only flow, but also return and balanced pressure in all rooms. When you change one thing it will effect another. Shutting, or rerouting, one duct should be OK although it's good to know the theory on how it's set up and what to watch out for. A very imbalanced system can actually use MORE electric than one that it set up correctly. If you shut off half your vents in rooms you don't use the blower will actually have to work harder under the pressure restriction and load. 'Not saying that's your case here, just a heads up.

A few portable AC unit examples:
portable air conditioner - Google Product Search
Walmart.com: 7,000 BTU Commercial Cool by Haier Portable Air Conditioner, Silver: Home Improvement

Another interesting option is a portable AC unit. Since it's dedicated to the area you can set it for the perfect temp. and just let it cycle automatically.

They are pricey but on Craig's List you can usually find them under half priced used. These babies use allot of power (about 1,000 watts) on full blast although if you just set it for 80F it'll be just barely cycling on low. Another thing these units do it put out allot of heat so you'll want to run the air exhaust pipe up on out with your exhaust fan. If you just lay it on the floor into a nearby room or area you'll just be spinning your wheels.

The one I got was a very inexpensive used price but the inside coils were very clogged with dust and hair like it was a mat I had to peel off. I knew it was not right because the machine would never cycle off and also leaked water. You see, the way these units are supposed to work is the upper coil, well call him Mr. Evaporator, is supposed to get cold and makes condensation. The lower larger coil, well call her Ms. Condenser, is supposed to get hot to remove the heat from the room via the exhaust pipe/tube. What goes wrong is the evaporator drain tray clogs allowing water to overflow and drip down anywhere it wants to. Proper operation would allow the water to spread and trickle down (like Reganomics) onto the condenser where it will help cooling efficiency and also evaporate and go out of the room via the.... You guessed it, the exhaust pipe tube.

And that's all I have to say about that.