It's solid enough.
I made the skirt for the other side today, and found some plastic (moulding) to use to for the mounting brackets instead of wood scraps.
I dremeled the plastic pieces to mate nicely with the shape of a bead of hard sealant or caulking that's on the underside of the 90 degree lip, so this side grips even better.
Pictures explain it best:
Above: this shows the shape of the bead in the other wheel well where I used wood.
Oh -- and this wheel skirt mod added another mod in its doing: I also deleted the rear mud flaps. (The front ones came off a while back.)
Next up... I think I'll work on modifying my DIY receiver to mount the Civic tail.