[chipinmom]

Either a hammock or just a complete cover is fine. I know they can be found literally everywhere, but I'm having a hard time finding one for backseats WITHOUT headrests. If anyone knows of anywhere online I can find one, I'd be grateful. Needs to either be in Canada or ship to Canada.

Would also like it to be waterproof

 

[Shell]

I have the "ASPCA Rear Seat Cover" from WalMart. It is waterproof and while it has little straps that can go over the back seat headrests, it has two hook also that attach to the "LATCH" attachments (where a child's car seat would hook in on the back dash area). Cheap too!
Durable, machine washable, has a place to bring the seat belt bases through it or not (closeable with velcro)

 

[chipinmom]

I have the "ASPCA Rear Seat Cover" from WalMart. It is waterproof and while it has little straps that can go over the back seat headrests, it has two hook also that attach to the "LATCH" attachments (where a child's car seat would hook in on the back dash area). Cheap too!
Durable, machine washable, has a place to bring the seat belt bases through it or not (closeable with velcro)

That sounds perfect. But It's not available in Canadian walmarts and I can't find it online anywhere that ships to Canada

 

[Shell]

Have you thought about just making one?
You could use one of those "picnic blankets" that is waterproof on one side and usually something soft on the other side. Sew attachments to suit your vehicle and cut slits for the seat belt base.
Water resistant outdoor fabric is another option; like the kind used for seating on boats.
If you use "nice" fabric, use an old (like garage sale cheap) bedsheet as a template. Cut the sheet to fit, try it out for a couple car rides to see if it needs adjustments and then cut the good fabric to match.