Yeah, the 45g and 55g standard stands are a PITA to design sumps for. Your choices;
One 10g tank
Two 10g tanks connected via bulkheads
One custom tank which would end up around 20 gallons
The pros and cons to each should be relatively obvious. Otherwise, you can modify the stand if the tank does not yet have water in it to fit something like a 20 long in there. You'd have to re-inforce the top back bracing of the tank, remove the back center brace, fill in the gap in the base of the stand with boards that hang out the back a little, and then you could rest a 20g in there overhanging out the back. That option's a little involved, but less involved than building a custom sump tank...