Uncle buck I'm trying to make a rubbermaid growbox do yuo think these things are really worth building and do you think one 4 inch fan with carbon filter on it be enogh to cool off. A bout 260 watts
the question 'is it worth it?' depends on a lot of things: the riskiness of doing so in your particular state, whether you are doing it for fun/experience or planning on doing it to make bank...etc. only you can answer that, but i have indeed seen a few awesome grows here from these boxes.
now, 260 watts in a rubbermaid is going to put off a lot of heat....it sounds like you plan to use 10 of the 26 watters that are available in many places. the key to whether a 4'' fan can cool it off depends on the strength of the fan.
inline duct booster type fans, which are AXIAL fans, will likely not be sufficient to cool it off if it blowing through a carbon filter. axial fans do not handle static pressure (resistance) very well.
this is the type of 4'' axial fan you would find at home depot:
http://www.comfortgurus.com/product_info.php/products_id/562
however, if you use a 4'' impeller type fan, that might be enough to cool a space like you have. i have seen 4'' fans that generate up to 171 cfm, which would be sufficient for a rubbermaid with 250 watts inside and a carbon filter outside.
this is the type of 4'' inpeller/centrifugal fan i just referred to:
http://www.bghydro.com/BGH/itemdesc.asp?ic=AEFCPVL04
a rubbermaid is what, about 2'x3'x2', more or less? so that is only about 12-15 cubic feet of space. if you want to keep a small space like that cool with 260 watts inside, you will need to change the air about 3-4 times a minute.
so, being conservative: 15cf x 4 changes per minute = 60 cfm MINIMUM
since the carbon filter reduces the cfm of the fan by about half or so, you need a fan capable of generating 120+ cfm.
my tip for finding one cheap: buy used! you may not be able to find one of those 4'' 171 cfm deals i linked, but sometimes you might find a good bathroom fan or oven hood fan capable of generating that kind of cfm. check craigslist, local home rebuilding centers, junkyards, whatever. you can find good deals!
ventilation is key, however. if ventilation fails, it gets too hot and/or the plants suffocate because they are not getting fresh air from which they can draw co2. you can almost NEVER under do it when it comes to ventilation.
good luck!