I've wondered if the GX550's would fit, as it extends downward a bit on the sides, and looks better than Toyota's. I've also contemplated just cutting the full diaper that will never be installed while I own it.
Instead of a cover, consider an attenuator which will also give you rear end protection. I used one from Sparebumper for years on my Sequoia. The one I had was a flat plate. It looks like they just sell the step mount now.
It must have been intimidating for other drivers, because I saw people not following as closely once I installed it.