Have you considered a media basket or even a HOB filter with media basket? StevieT has awesome products, and his products have an excellent fit 'n finish..
http://shop.mediabaskets.com/
Or have you considered a small reactor like such?
http://www.marinedepot.com/Two_Littl...FRISBP-vi.html
http://www.marinedepot.com/ViaAqua_P...FRISPR-vi.html
Can't comment on the quality, but they seem like popular choices.. Hope this helps.