Not really uncommon that people use fibreglass rods, do a search here or on other boards. I don't make any claim as to suitability, just saying people have used it instead of acrylic. I guess fibreglass boats have a gelcoat.
Another option is nylon rods, also something IPP should carry. I don't know if it's a cost-effective alternative though as I've never looked into it, just noticed that they had them last time I was there.
Also generally speaking the rods should just be used for a little stabilization. I think anything used as a load bearing structure will likely weaken over time, so it would be better to find the way the rocks fit together best and use the rods as a guide. Just my $0.02.
