Non-chemical plastic container. Cut-out and shape with a blowlamp. Frogs love it.
What a good idea! How big is it, and have you sunk it into the ground?
This one is about 25 ltr...but the bigger the better. Yes, I dug a hole for it and made sure entry and exit 'legs' suited the height. Dead easy and a life saver for more than frogs. Had lots of dragon flies over this one.