I'd look into making them a tin and wood structure. It could be partially set into the ground, mulched heavily on all sides, framed in and have a heavy screen/hardware wire lid or lids. With tin you'd also have less to worry about from raccoons. Maybe even line the bottom with wood to prevent digging. Inside you could apply expanding foam and concrete to create insulation and visual interest.

You could also include one or two small, preformed, ponds like a whiskey barrel liner, or a "flexi" form like the Willowfly. That would create an area large enough for soaking -and- spawning.

Then again, I like huge projects. Other folks might run screaming from something like that.