Dinghy Chaps do a few things. First, they assist in extending the life of the dinghy fabric. A dinghy that lasts longer saves money. And second, they make sitting on the tubes much more comfortable. A dinghy in the tropical Sun will get hot. Unless you like roasted ham; dinghy chaps will assist your travels to and from the yacht are much more comfortable.
There are dedicated Canvas workers that specialize in Dinghy chaps. If one is around start there. However they may not want to do some of the work you wish so be careful. In Bali we had a new set of chaps made and neglected to provide enough detail. We needed to get them modified in Malaysia.
There are a few areas to concern yourself with in creating a set.
First, a Bow Bib. This piece of the chaps greatly assists in keeping the cover down in the bow and out of you eyes. A lower tie will help. Run the draw cord through the lower edge all the way around to the stern. The keeps the chaps nice and snug. This, we neglected to do on the last set. Maybe, just maybe; our next set will be perfect! Some Chap canvas people will want to glue on velcro and keep it tight to the edge above the rub rail. Don't do it!
Second, the reason you don't want to velcro above the rub strake is that the Sun will degrade the rub strake and pieces will break away. See the last image.Protect as much of the dinghy as you can.
Third, chafe protection. As you can see from the images on the right you'll see places that chafe. We tried using the traditional Sunbrella but that didn't work real well. Use something more resilient, a vynyl for example. Leather while tough will also not last long due to the Sun and water.
Finally, be careful of color. While dark colors may look good, they also pick up the heat from the Sun. Remember, don't cook the ham on your dinghy tubes. Additionally, if you are locating your dinghy in the dark, make it stand out, not hide.
Good Luck