The best diffuser I've made ran from a power head. I have made a diagram but still have to get it hosted - Here is the instructions. If you do it right you can dissolve 100% of your c02. I'm pretty proud of it acutally.
1. Cut a small piece of UGF
tubing(The clear uplift pipe) - about 3-5 cm long and silicon it to the under side of the lip of your tank. IT should be airtight and extend below the water line.
2. Attach the outlet from your c02 generator bottle so that it bubbles into the the UGF
3. Attach a venturi fitting to your Powerhead (This is the one used to blow bubbles) and have an airline tube running into the the UGF
tubing. This will suck the CO2
into the power head without putting any pressure on the generator bottle.
By itself this will work pretty well, but if you want to disolve the CO2
4. Take two peices of UGF
tubing about 6 cms and 10cms. Cut a hole in the middle of the 6 cm piece and round off(inverse) the top of the 10cm tube so you can join the two peices into a sealed T shape. The 6cm piece will form the cross peice and the 10cm the vertical. Seal one end of the cross piece (I found a sports drinks that had this clear plastic cap that fit perfectly - Nice drink too). Now attach the open end of the cross piece to the end of the powerhead.
You might need to play with the length of the vertical piece, or even pack it with foam to get the c02 to dissolve completely(Avoid foam if you can, it will harbour anoerobic bacteria) The power head will blow the C02 and water in the cross peice and literaly create a hurricane that dissolves c02 like you won't believe. This system has the added benefit of ensuring that you yeast/sugar mix cannot end up in your tank.