In reply to Petarghh:
Bleach bottle plastic and some bits of coathanger wire or cable ties.
Cheap bleach (white bottles) is less than 30p a bottle, but the plastic is bombproof. Yellow or blue costs a little more
You'll need two bottles.
Cut the flat piece of the bottle out with scissors after cleaning your loo to within an inch of its life.
Cut to fit with scissors. A slightly iffy fit is OK - slightly oversize is actually better as the bulge compresses under weight and then pops the snow off when you lift your foot up.
Punch smooth and even holes for the ties with a standard paper punch. Make some little clips of wire to hold them on. Cable ties are OK, but they can get brittle - better to put a little more effort in and use wire.
I've never bothered with the heels personally.
My current pair has survived about 40 days on the hill so far and there's no reason to change them yet.