I doubt I am the first to think of this but may be worth a mention.
I have a make shift twenty gallon long sump that holds my skimmer heater and a extra hob
filter and return pump for DT and fuge. As we all know you cant fill the sump to capacity in case of power loss. I never measured it but my plumbing holds easily two gallons of water. Kind of a waste of space.
Today I bought a rubber made container slightly larger than the twenty gallon sump. Emptyed out the sump, water change day any way, slid it down into the rubber maid container and refilled. Now I can fill the sump to capacity and in case of power outage it will overflow into the rubbermaid instead of floor.
Also takes any guess work out of top offs. Now just add fresh water until sump is at capacity.