I pretty much did the same as cyclopath, except I watched Ebay (a gamble) for my filter.
If I remember correctly, its about 4 or 5 inches thick. I just matched the suggested CFM to the filter and made a nice well sealed box. Need a pre-filter that can be replaced. You can use a Rheostat Switch switch if you fan is too powerful.
When its all done. You should be able to hold a lighter up close to the filter and the flame not blow more than 45 degrees, preferably slightly less.
If you have bad spots in the filter, you can use silicone to mend small holes.
Larger the better, if you plan on doing a lot of jars. This will save you a lot of time later in the game.
Iv seen a lot of good deals on this site for flow hoods.