How deep should the water be for these guys?

I've seen some sources state no more than 12 inches, others say at least 12 inches and up to 30 inches.
What is the water level in your tanks? I was thinking around 14 inches. Would that be ok?

Also, I really don't want the frogs escaping from the tank as I have 3 cats, and I imagine they'd find a frog before I even notice it's missing!
Does anyone have any tips on keeping them in?
I know you should have the water level lower than normal, and cover any holes (how big a hole can they get through?).

I am planning on putting a large hollow piece of bog wood in the tank which is 10 1/2 inches high at the highest point. Will that make a difference?

The tank is 69 UK gallon size and about 22 inches high.
I haven't set it up yet, I've just been cleaning it out tonight because it's been in the shed & was an ex marine (given to us by someone who didn't clean it first).

I want to make sure I have the set up correct before I fill it and cycle (the bog wood was used for a coldwater tank originally, but has been in the shed for a couple years so will need a good soak/rinse).
I'm planning on well washed play sand for substrate (my hubby uses it in tropical and marine tanks and assures me it will be fine), and silk fake plants.