Could put a wider bowl. The surface area of the water is what will matter but imo, if your room is 35% humidity, you need to find a better way to control the air flow in the rack as best bet from the look of the size, you can get maybe 50% more water surface area but I'm not sure that is going to be enough if you have to spray every 3 days. Also you dont want to keep the snakes on wet substrate/paper towels 24/7 as that is going to promote bacteria growth and could result in scale rot since retics like to piss and that mixed with a 24/7 wet paper towels.

As for a bigger bowl to soak in, go for it. Get something like a tupperware rectangular container that isnt too high and is wide enough to fit in the tubs.