I have the same problem, offside nearly worn away and the neraside is torn and worn. Just had a look under my mk iv and they seem to have a metal edge which fits with clips into the bottom of the bumper. I presume the best way is to drop the front bumper and fit them that way. I've just noticed I am missing a few clips and bolts holding my underpans in,looks like some work for me now.
Bets of luck, let us know how you manage it.
I recently wanted to replace the missing air dams on mine and popped into Lexus to enquire about parts etc
The plastic bits are indeed c£37 each but each side is held on by two metal brackets that cost £80 per side !
I decided to look for the brackets at a breakers before taking the plunge.
In respect of fittings, on looking at the parts screen and talking to the parts chap, it would appear that one long bracket sits inside the spoiler lip and has verticle teeth that poke down through the holes in the spoiler and then the other bolts too it from outside with a clamp/sandwich effect.
Now I haven't tried or seen these parts so could be totally wide of the mark but this may help.