Niamh,
I’ve frequently mentioned how important ramps is as a learning activity, so it is great to hear it from the classroom. My go-to-link for the pedagogy is this from the Ramps and Pathways team.
It is an ideal design and make, and we do it at BCO with pipe insulation cut in half, easy to source, reusable and flexible. Get it from your local builders providers, choosing the widest interior diameter and thinnest outer material to get something that is easy to curve and can accommodate a lot of different size marbles. Give different groups different colour marbles, so if they lose their marble you can check that another group hasn’t collected it before you move every item of furniture looking for it. (that’s from experience!).