The sump can only be appx 1/2 full(to allow for backflow in case of power outage) so you would have to have a 20g container in order to have about 10g of actual water. Some lfs
will sell their old display tanks when they get too scratched to use in the store. They usually sell them dirt cheap(maybe even free if they know you). My sump is a 10g tall trash can but we have had a sump/fuge combo out of 2 rubbermaid tubs plumbed together with pvc
. Each tub was about 20g and were only about 1/4 to half full so we didn't have the bowing wall syndrome. Plus we cut out the lids to allow for electrical and return lines so the lids helped to hold the sides solid. We cut the lid on the refuge and glued plexiglass on top so we could put a plain aquarium strip on top for the algae to grow. It can be done many ways so don't be afraid to experiment. Google it.