what kind of filter do you have? if you have a wet/dry filter, you can put the rowaphos in a filter bag and then just lay it right on top of the drip plate. you can also probably get a cheap hang on back filter and put the bag of rowaphos in there (not sure on this one though, but I don't see why it wouldn't work).
do you know why you have a lot of phosphates? you aren't using tapwater are you?
also, be sure to change out the rowaphos as directed as it will leak phosphates back out over time (or at least that's what I've always heard).