Some time back I acquired an unused Ontario Pilot's knife in a batch of knives. It was not particularly sharp, had a thick edge and the grip and sheath were unsealed leather.
I didn't have much use for it so decided to leave it in my car. I saturated the sheath in hot bees wax and thinned out the blade edge so that it cut well and was easy to touch up. The grip was coated with epoxy until it wouldn't soak any more up and the blade was coated with a mix of lanolin grease and beeswax. The knife was been sitting in the spare tyre well in the car (in tropical conditions) for over 2 years without any sign of corrosion or degradation. Doesn't meet your synthetic sheath requirement but it is a good tough knife, the sheath is pretty secure, cheap enough to be lost without grief and capable of doing most things a knife should do.