What material boat bags are made of?

Just curious. What sort of material the bags are usually made from?
I need one specifically for debris protection during long distance drives.
Would tarp material be good enough? Don't see why I can't hand-craft a custom fit bag from a large piece of tarp.

I heard people have mixed feelings.  Some say a bag will protect however many say that a bag flapping against the boat will just scuff it.

The bags I've had were a nylon canvas.  Nelo uses a stretchy sock that looks nice.

Yeah the scuffing worries me for sure. Tarp may be too rough.

I'd like to have one of the nice custom made Sunbrella canvas boat bags, but I felt it was overkill for my need. I just got an Epic surfski sock which is made out of knit fabric material with a neoprene bow cover. I bought it primarily to protect my boat from UV radiation, bugs, and pine sap since my boat will spend it's summer in Maine. We'll see if it holds up for the season.

I think I will just shrink wrap it for the longer trips. I store it in a garage except a couple of weeks in summer when we rent a cottage. At that time just put the tarp over...

I just put the Epic sock on my V8 Pro, it's more environmentally friendly than shrink wrap and seems like a cost effective and decent product.  It definitely won't scratch the ski. 

quck google search :

australianpaddlesports.com.au/product/su...tch-cover-surf-aids/

eliteoceansports.com/2014/new-product-bo...or-surfskis-and-k1s/

I have also been using a knitted boat sock, albeit one from Reed Chillcheater.
The knitted fabric is very soft and did not cause any rubbing marks or scuffing during several 100miles trips.

I kept my surfski in the sock all year round to protect it from UV and tree gunk and that seemed to work quite well for the last 2 seasons, provided one rinses the sock from time to time. The only downside was pulling the sock over the boat every time I wanted to go paddling (which, according to my wife, is rather often...).
Therefore, I recently upgraded to a sunbrella  boat cover by Burnham Boat Slings.
Looks beautiful, included a tail flag for transport and opens/closes quickly using a full-length zipper. The cover is padded and fits perfectly over the boat, without excess fabric, so I wouldn't except any flapping and scuffing during transport.
Bloody expensive though, especially since I had to pay extra import taxes here in Belgium. So, I don't know if I would recommend it to others. YMMV.

Sounds nice. Not sure if my use justifies the expense though. The boat will be indoors except 2 planned trips during the summer. For my regular paddles I won't need it at all.