I'll need to admit I am not familiar with pondless ponds, but I would love to hear more about them and see a sketch of what you had in mind. I have seen pondless water features, where a waterfall is re-circulated from an underground retention pit, and there is no visible body of water.
If you'd like to share an image with our members, I trust they would be more than happy to advise on how you could make one.
I want to build a pondless water features, l can do the stream just wanting ideas on supporting the rocks for the surface of the pondless pond.
I had thought about doing this down the side of my place but might just end up creating a dry creek bed instead.
Regarding your question it would depend on the size and shape of your water resovour and I had some thoughts maybe galvanised wire mesh, storm water drain grate, upside down milk crate inside a storage crate.
As long as it can take the weight of whatever is on top I suppose, and does not rust.
It's great to see you've already received a helpful reply from @Stuardo. I'd agree as I've always thought stormwater pits would make a great reservoir for a pondless water feature. Their grates are very strong and designed to hold weight, the plastic is perfect for drilling into to run your return pipes and there is plenty of space to house your pump. They also come in a variety of sizes depending on the volume of water you wish to hold. I'd imagine something like the Everhard 450mm Series 300 Deep Stormwater Pit Base and Everhard Class A Galvanised Grate to Suit Series 300 Pit would be suitable for a small to a medium-sized water feature.
Due to their solid construction, you'll be able to place a considerable amount of rocks on the grate section.
I look forward to following along with your project and please let us know if you have more questions.