Just thinking here it is likely a bigger pain than it is worth but if you designed a 'rodent' rack could you stack 2 tubs open end to open end and cut a screen opening on the 'top' one? That would double the height with the same footprint. If the rack was designed to pinch the two together it should hold fine. Just a thought I know little about rats but I just thought hey that might work... It will take someone familiar with the tubs and familiar with rats to project if it is feasible or not, I can't, not enough experience.