Space per rat?

[Bogertophis]

Definitely would not take long with plastic.... or wood.
For the suet holders...A design based around it isn't a bad idea. But in my experience, the holders they sell they tend to be kinda cheap and never quite close right. But it could just be the brands I'm familiar with? Also you need to be careful about how wide the bars are. Some of them aren't very close and I've seen a young guinea pig get stuck in one once when we tried using it to keep the hay a bit neater at a pet store I worked. Nothing serious though, we just had to help him squeeze his head back out after he calmed down

I hesitated to suggest the suet holders, because as you said, most are cheaply made -or coated in plastic which will get eaten by rats, & I'd also worry about them cutting their mouths on sharp wire edges that stick out. As far as rats opening one, that's an easy fix though- the squirrels here used to open mine* until I used a wire twist to "lock" them. A paper clip, key ring, spring clip- many things could be used to keep one shut. (*I'm not using them for rats- I feed the birds in winter & the squirrels are thieves. )

This is the sort of reason I like to stroll thru home improvement stores, looking for ideas to use things creatively.