does ne1 have a water proof saddle or is it possible to modify a saddle so its water proof?
put a plastic bag under the saddle cover
plastic bag under the cover will help always but most saddles should be water resistant by them self like a motorbikes seat.
the new koxx one kevlar saddles are very water resistant from what i hear.
Why do you want it waterproof? Do you just want to keep rainwater from the foam?
Bump.
The KH foam is closed cell so it really shouldn’t hold any water. You would just want to dry the cover.
Spray it down with rain x.
you should ask those guys 
the koxx saddles (as long as they dont have any tears, are water resistant.) I’m pretty sure the kevlar makes no difference to their waterproofing. The kh have kinda textile on them the koxx have fake leather all over.
Does it matter ?
I have ridden in the rain sometimes. It’s really warm here now, so it’s just fine.
I don’t have to do it every day, like in Blighty maybe. Still, it just doesn’t seem a problem. Cosed cell foam on my nimbus and KH saddles. What kind of trouble are you having with rained on saddles? Maybe it depends where you live. Mine seem to absorb little if any water, I just ignore it and everything seems fine.
I think you will ruin your bearings/rust your frame before the saddle.