I've been asked to make Deuce tarps and have thought about the material and work involved. Here are some rough calculations. To get over edges of the bed, with a standard low stake bed as pictured in your avatar, one needs roughly 170-180 inches, or 14-15 feet of width. Front to rear, the bed nominally is 12 feet long. This doesn't leave any amount of overlap so a 12' tarp wouldn't want to stay in place likely and would be leaky at the front and rear. Add two more feet for that, so 14 feet.
For minimum coverage, I'd estimate 14'x14'. TarpsNow lists many options for material, and makes custom dimensions affordably (relatively speaking). Prices for a 14ft square tarp range from $200-1000. Decent materials are in the $400-600 category. You can get an untreated 10oz weight canvas tarp in that size for $175 plus shipping. I would not recommend that weight, or untreated, as it won't last long. You could treat it yourself with naptha-thinned silicone caulk and it would be more weather resistant, but you'd need to reinforce the wear points. $300-400 could get you a heavier weight "period correct" looking canvas tarp that you can dye or paint to match your desired color and then silicone to seal. I would expect maybe 5-8 years if taken care of: minimize water pooling and keep it treated and wear points padded.
As far as "right now" options, the closest size at Harbor Freight is their 16 ft. x 20 ft. Silver and Black Extreme-Duty Weather-Resistant Tarp. The double laminated polyethylene will NOT stand up to UV for very long, but may get you two seasons. It costs $50 near me. Once the fibers begin to de-laminate, I am not aware of any treatment to correct it. It is disposable.
I cannot make a tarp that large, due to shop limitations, or for the price TarpsNow charges. They get huge bulk discounts on fabric, clearly.
Hope this helps,
Bulldogget
