Several of the Rev War groups have patterns/drawings on their web sites. The NWTA used to have patterns. But I have not looked at their web site in a couple years.
Also, the Sketchbook 76 has a measured drawing of a haversack in it.
Historically, a haversack was a military issue item designed to carry the soldiers food rations. Every couple of days each soldier would be issued raw food for the next several days. Meat was issued RAW, and peas/grain/oats was issued un-cooked. This was all placed in their haversack to carry with them. At night, a "mess group" of 6 to 8 soldiers would get together and cook up a combined meal from their issued rations.
So that haversack would have had raw meat carried in it. So cloth was the chosen material.
Nowdays, a haversack tends to be a catch-all shoulder carried bag. It often is used to carry clothing and gear. Any food carried in it tends to be pre-cooked, or wrapped up well in other material. So ... ooozing .... raw meat is less of a concern with modern usage of a haversack. Thus some people decide to make them of leather for more durability.
One other item you may want to look at is a Snapsack. It was sort of going out of fashion by the Rev War, but was also a sometimes military issue item. It basically is a cloth or light leather cylinder with a single strap attached to the top and bottom of it. The top usually has a drawstring type closure. The bottom is either sewn closed with a round piece of the same material, or closed with another drawstring like the top. One fancy original leather snapsack also had an extra pocket sewn on the outside - with tied flap closure.
Choice of thickness of leather is a matter of personal choice, and how much weight you wish to carry. Most go with thin/flexible leather - like deerskin. It acts a lot like thick cloth.
And then there are the little "design" differences. Most haversacks are a simple flat rectangular pouch with the back folded over to form the flap. And a simple strap attached near the top of each side. But one original had the strap sewn to the top of each side on the back, but the stitching went completely through including the flap! Most just get stitched to the back, possibly with an extra reinforcing piece inside. And some people have made their haversacks with a gusset down each side - forming more of a full box instead of flat envelope.
Just a few rambling thoughts to share. Hope they help. (And trying to avoid that whole discussion about a non-military person using a "haversack".)
Mikey - that grumpy ol' German blacksmith out in the Hinterlands
p.s. Something you might also want to look at is a Market Wallet - a long thin bag sewn closed at both ends and with a hemmed up slit running lengthwise in the middle of one side. You stuff your gear into each end, and the drape it over your shoulder. The weight then keeps that access slit closed. Originals were made in many sizes - from small enough to lap over your belt/sash all the way up to "saddlebag" size for packing on a horse.
