Plastic bottle caps, yep. I've heard of people using them along with the cut-up straws. Surface area is the goal, but cash is king, so why not use something cheap & easy to get? The more different types of surfaces in there the better.
My sump is still in the testing stages, but I've had good results using that plastic knitting mesh as a strainer in the first chamber, before the sponge & other mech. It doesn't restrict flow much at all & grabs all the big stuff. I'm thinking of cutting another sheet on the diagonal so I can have a few layers with offset holes. For bio, use anything plastic or foam thats cheap.