They set that tank up more for a terrestrial frog ( tinctorius azur dart frog). The 40 breeder I wouldn't think would have the optimal height for tree frogs.

A water feature can be accomplished several ways. Google custom vivarium water feature and check out the videos. You going to create 2 types of environments within your tank seperated my a water proof boarder. You could probably use pond sealant foam used for outdoor water features. Usually the false bottom is approx 4" covered by a mesh barrier then abg mix then leaf litter (darts) or moss. You also typically use springtails and isopods in the soil as a cleanup crew (you have to culture these but not hard). This sounds daunting but isn't that complicated.

The water side you can use what ever for substrate. However you will need to find a place to hide your pump filter heater etc but you can still access it if need be. You will also need to leave a place to siphon excess water from your vivarium side. If your substrate is constantly sitting in water it will rot and smell like a swamp.

If I was going to do something like this I would use a 55 gallon plus setup with a sump. That way you can keep more water volume and hide all your equipment. In that small of tank the juice is worth the squeeze. In that youtube video linked I bet you would have to change the water dang near everyday to have acceptable nitrate levels