I bought a second-hand magazine rack for the rabbits' hay, and works quite well (the idea was that it would keep their hay in one place, but they couldn't sit on it (silly me!) - rabbits won't eat hay that they've used as bedding).
The rabbits and guinea-pigs are currently sharing the same shed (but it's sectioned-off), however, I have on occasion gone in to find a swarm of guinea-pigs on the rabbits' hay rack! They seem to love it; they can sit outside the rack and eat hay, but also sit inside it (hidden underneath the hay) and trough. Probably not what you're looking for though, as I imagine that you don't want them to be able to sit on the hay. However, if you could put, say, chicken wire round it to stop them from getting in, it may work. Just a suggestion, as they do look very cute when I see them on there - I'm tempted to buy another one just for them.