Your res will hold more than 10 gal, prob more like 15 or 20. You could probably use a 25 gal, but I would just snatch up one of those 30 gal storage containers from Wally Mart. They are about 2 /12x1 1/2x15 inches High. Throw it right under the tray IMO.
PS, friend uses a 4x4 tray and his 45 gal res hardly covers it well enough, so he thinks.
Adieu