For variation, I've made hay boxes before (out of old cardboard boxes with a cloth cover on top), which one or two pigs would always love. Always at least 2 holes in the side so nobody got stuck in inadvertently, no squabbles. At some point the bottom of the box would get pretty soggy and i'd toss the whole lot dirty hay and all. There would always be hay elsewhere in the guinea landscape though.
Once I made my own hay out of lawn grass, sun-dried, turned properly and everything. I had it stored in a box in my living room which my guineas Poppet and Robbie discovered while out for their run. I hadn't thought they'd be able to get in, but they did find a way. They loved it. Lovely place to hide in and of course pee and poo. The first few times they were really quiet in there, so quiet I went looking for them everywhere, including throwing a glance in the hay box, no not there. Then had a proper look: oh there you are! I suppose you know you're not actually meant to be in there?! After those first few times, I turned a blind eye and they became more conversational - it was just part of their landscape when out for a run.
I have heard of people turning old wool socks with holes in the toe and heel into a hay sack, hung on the bars of the cage, back when cages were smaller and had bars (which they don't tend to have in Germany any more, guinea slaves make huge landscapes out of wood and plexiglas mostly, rather than using C&C, tho you can get the latter). For me that kind of hay sack meant less hay dragged around the floor of the cage getting dirty so I supposed less hay usage, means less hay needing to be procured, which was always rather a chore due to my not having a car... Plus I thought it meant some variety for the guineas, and maybe it was. Otherwise I'm sure all the hay piles you see upthread with guineas playing hide-and-seek in them are the way to go!
What generations of my piggies loved were wide drain parts from the DIY store stuffed with hay. They were often drain parts with 3 openings instead of 2 and so each pig would sit at his/her own opening, munching hay and moving further and further in as munching went on. Eventually most of the hay would be eaten and piggies would wander or even run through the drain part, making a tunnel. They were hard plastic and the only downside was that an occasional pig thought it might be good to chew, not that they got far with that because it was hard. I can't say offhand how wide they were, but definitely wider than even a chonky pig - they could all run through a drain section with no problem at all. Not much longer than an average pig either.
They much preferred big piles of hay on the cage floor