I was thinking of something more along the lines of that flexible black plastic garden edging you see in the Lawn & Garden department. It's got a rounded edge on one side and a sharp straight one on the other - you could drill and then mount the rounded part to the underside of the front bumper fascia with a few screws, trim up the edges to make it look cleaner, and call it done for pretty cheap. Not sure how it'd hold up to heat, but it'd be a little more rigid than rubber/vinyl, anyway...
I don't see any reason why you couldn't try it but there is a HUGE difference in rigidity between the garden material you mention and the ABS plastic that the front spoiler is made of (ref. the pics up top). The plastic is very rigid and doesn't perceptibly change shape in the summer although it does become more pliable. The garden surround you're talking about, I don't think it would be rigid enough to stand up to any speed.