Going to dismantle... learn from me!!

[UncleChester]

An alternate to drilling is to push the substrate and drainage layer away from a corner and build a dam so you always have clear access to the drainage.

Pictured below is one made out of river rocks held together with a dab of silicone between each rock. The 'dam' needs to be porous enough to let water through but tight enough to keep critters from crawling into it.

I have added a dam like this to a tank with a layout like yours after it was inhabited. I built the dam outside the tank and actually installed it without removing or even disturbing the frog- it only took a couple minutes to push the substrate out of the corner and remove the excess hydroton I had as the drainage layer, then put the dam in place. No silicone is required to hold a dam like this in place (apart from what's holding the rocks together), it's made to fit in the corner and it's stuck between the substrate and the glass so all the curing can happen outside the tank.

This also gives the frogs access to the drainage area as a pond.

A built in drain would be pretty convenient though. You could always buy another tank and build it from scratch with a drain added. Then move your frogs over. Then rebuild their old tank. Then get more frogs.