Go to a military dump, buy one of the military bivouac/bivy sleeping systems. They're like a winter sleeping bag with a tent canvas bag around it. Very sturdy and dry, easy to fix yourself, should be affordable. They cost ~$250 new, so find a second hand one.
Some examples:
https://www.trailspace.com/gear/u.s.-military/mss-modular-sleep-system/
https://goarmy.co.uk/products/british-army-3-piece-modular-sleep-system
https://www.amazon.com/MT-Military-Sleeping-Woodland-Multicam/dp/B095H67CKB