Just installed new bumper fillers from G Body. Install fairly simple. Adjusted a couple of spots (mostly rear). Turned out ok, not show quality but definitely good enough for my driver.
I asked the question on fibreglass or plastic to Brian at Gbody parts and he pointed me to fibreglass. Mine fitted with no drama and have sufficient flex to allow you to get them to fit tight.
When I did mine I just unbolted the filler and removed it and had enough room to install the new one without unbolting front bumper or headlight bezel.