Well in a pinch you COULD get away with a 10LX4WX3D but is not going to be an ideal home for your RTC
Just keep in mind that your 120 is only 18" wide so once your RTC is over that length it really does need a new home... your average RTC will break 18" inside of a year but that is depending on how often you feed, what you feed, water quality and such they can grow faster or slower... but the average will be by it's first birthday it'll be getting a new tank as a gift LOL