Don't use underlayment. It eventually falls apart. I put it under my liner a year aao and in the spots where I have pulled the liner back to make adjustments the roofing felt was in peices and crumbles to the touch.

Pond liner scraps would work, maybe a thick plastic like what is used for bamboo barriers.

Thank pcan! That's very helpful. I'll steer clear of underlayment.

Still, I'm wondering if there is a more robust product out there that is similar to Pond Liner in terms of durability, but less expensive to manufacture because it doesn't have to be non-toxic to fish.

I use the black barrier stuff for gardening for things like this. I can't think of the name of it off the top of my head, but it is non-woven, water can pass through it and you anchor it with u-shaped pins. I built a stone "planter" around my skippy filter and put this underneath and behind the stones to keep the grass out. Has worked great so far.

I try to keep very aggressive maple tree roots out of my veggie beds & just used metal flashing on the inside of the rocks, top edge under the soil. It was pretty cheap & it's certainly durable. The annoying tree roots work hard to go over & under it, but I don't think grass would do that. Honestly, any non-rotting barrier should work for you, you just want to be able to hide the thing.