I bought some 'build it yourself' adjustable wings from fleabay and painted them with a (nice?) brown wood preserver. I have then used a variety of poles and in fills, from just plain wood poles, unpainted, to rough branch poles cut (with permission) from a coppice wood. I have even used garden type trellis, and worked some evergreen type brush into it.
I did try a blue tarpaulin with a frame around it, to try to imitate a water jump.... he wasn't fooled for a minute, but jumped it anyway
Use everything & anything as fillers! I've used planter trays from garden centre for water trays with yellow rubber duck floating on them! Milk churns, shavings bags (bed max) I've got a gate filler & put one of our for sale signs next to the wing, scarecrows in the wings, life size stuffed calf by the wings....... You name it! It's quite fun once you get stuck in, just make sure it's all safe, not trappy.