(The diagram doesn't show it best but 4-8 holes in each row will work best for best circulation. jst make sure not to degrade the integrity of the pipe so it stays sturdy. Four 1/4 inch holes or 8 1/8 inch holes or something inbetween should work best. Use your judgement but as long as the tube stays strong, I don't know that you can put too many holes in it.)
The tube can be either an uptake tube from an undergravel filter a piece of PVC pipe. The length of the pipe can be customized to whatever you want.
#1 Cut the pipe to length
#2 Fit the sponge(s) to the pipe. I recommend at least 3 sponges so you can rotate them out to clean them. Round will work better. Cut the sponge so it is about 3 inches in diameter. It can be narrower but don't go too wide or circulation will be hindered. It will also work better to cut actual circles out of the sponge just smaller than the pipe so it fits tight and there is not much compacting of the sponge to block water flow.
#3 Fit the sponge(s) again and mark the top of where the ponges fit moving them up about 1/2 inch from the bottom.
#4 Use either hot glue or silicone to fill the bottom of the tube to about 1/2 inch. Place some wax paper under the end so you don't make a mess and it can sometimes be removed fairly cleanly.
#5 Drill 4 or more 1/4 inch or smaller holes around ones section of the tube starting just above where you filled to just below where the sponges come to on the tube. Make at least one row for each portion of sponge used but the more holes the better. (See diagram)
#6 Cut a length of air tubing as long as you need it to get to the air pump and down into the bottom of the tube. Connect an airstone and turn on.
Adjustments may need to be made to the sponge thickness and tube height but only time will tell as to how clogged the sponges might get. If you find that there is not much mulm collected on the sponge you might need more or larger holes or need to turn up the airflow or need to cut the sponges down a little to improve circulation.
Please add to this, the more options a person has the better.