So before I bought my reactors I wanted to try and make one and this is what I came up with. Not the best design, but with some minor improvements it would work well. First improvement I would make would be the tube going into the reactor needs to be rigid (I only had flex tubing at the time). Second would be the size of the barbs and tubing ( I would use 1/2). 3rd would be a larger pump (I used what I had lying around)
It worked when I set it up, but with the suggested improvements it would work much better. Phosphate reactors aren't expensive to begin with, but for those who like to DIY everything, here you go.
The cylinder is a cracker container from Linen n Things ($5) and the sponge is just from a canister filter.
If anyone tries to make one, please post some pics of the final design.