Basically impossible now, the tiling flange is in the wall. Just going to have to build something around it, I have a feeling any flexible material you would wrap to follow the contour of the base will look like crap
I'd recommend getting some form styrofoam...the same stuff used in construction around exterior window casings, then saw cut 2" x 8" and mastic them around the base in a vertical direction.
Here is a pic of a similar look, which is shown on the steps leading to the tub.