Definitely coir/coconut fibre. The blocks you get from Bunnings etc and soak in a bucket of water, til the expand and fall apart. However be aware that many of these have fertilisers added which you wont want with your maggots.
I tried breeding them once and am now officially banned from attempting to do so again at Guru HQ, upon the threat of immediate divorce.
My wife is actually a decent looking bird and looks after me pretty well . . . so I reckon I'll leave it to you guys moving forward