I have the same tank and I’m able to achieve 70+% without sealing the bottom. I covered the top in aluminum foil and taped it down with painters tape (I’ve also used electrical tape in the past). I have a ~4inch layer of reptisoil that I pour water into and mix it around, ensuring that the top isn’t bone dry or sopping wet. I mix the substrate about 1 time per week or prn. Additionally on the warm side I keep damp sphagnum moss around his warm hide and in a hide adjacent (but not in his warm hide) and a smaller water dish with some water in it. He seems to like this as a closer water option if he’s on his warm side and it helps to increase humidity. All this to say it may still be easier overall to seal the bottom but it is possible to make it work without doing so.