My young royal is in a rub about your size, but hes also inside a 2x1x1 vivarium.
He has a hide on the side with the heat mat and his water just overlapping it to evapourate and gain some humidity ... its spose to be about 50% i believe as there sub-tropical.
its hard to get a warm and cold side in a small container, but with my larger royal in a 3ft viv i try get 87 hot end and high 70s cold end. but ambient temp around 84 will be suitable i guess, and night ambient of about 76.
I also crammed in as much artificial plant as i could and he seems to be thriving in it, its almost dense to the point were if hes hiding at the back i cant see him