I am building a HR II and also decided more fuel would be nice. I designed a removable tank to go in the rear baggage area, it is a rectangular box shape but is tapered to match the seat back angle, and holds 16 gallons. A small electric fuel pump transfers fuel to the left main when there is room. I installed piano hinges along the sides and front of the baggage floor and matching ones on the aluminum tank for easy in and out. Just connect the fuel drain line and vent.
I figure I will not be doing long leg flights with a passenger when we will need the entire baggage area, and when extra fuel weight in the baggage area would put us out the rear of the CG envelope. But for solo trips 100 pounds in the baggage area is OK. I can often make out and return flights without having to buy fuel. I am a firm believer that human bladder capacity has no relation whatsoever with the frequency of airplane refuelings!