Up-Flow 20 gallon Filter Media?

[Jason]

I have decided to make an Up-flow filter out of my half barrel spillway liner, I bought lighting grate and cut out 2 levels with the first about 6" from the bottom and the next about 7-8" above that. I have a 1" 90 elbow coming out of the bulkhead about 2" from the bottom for the "Swirl Effect". I put about (2) 5 gallon buckets worth of bio balls but can't figure out what to use for mech media? Floor buffer pads are only 17" wide, the blue air filter sheets are 20"x30" and the level I need mech filtration is 23" wide ? Thanks!