Looking for ways to block off a hedge without hindering hedgehogs

The boundary between my garden and my neighbours is a long hedge, approx 20m long. There's roughly a foot of clearance between the ground and when the branches form a solid boundary. I've been steadily turning my garden into a wildlife garden, but due to the gap under the hedge the local moggies love using it as a cut-through. I want to block off this gap to stop them getting through. However we have hedgehogs visiting the garden who presumably use the same gap so I don't want to stop them entering. I'm thinking of some kind of barrier with regular hedgehog-sized gaps. Any suggestions? Some points:

- Netting/wire is out as it's too risky for the hedgehogs getting tangled in it.

- The hedges are very popular with nesting birds so anything too tall would be harmful to them.

- The sheer length means it needs to be something cost effective.