If you are determined to stick with a 12 ft you might not need a blind. Some BlindGrass mats laid over the boat, you and the dog should be good enough concealment in most situations if you sit down.
A tanglefree panel blind actually might work and also give you ability to hunt on land too. Add natural cover to it.
If you make homemade your priorities should be that it is light and easy to set up and take down. Figure out the dogs spot after you build it. Lots of blind plans out there. Scissors aren’t too bad if built right.
12 foot is a small boat for a boat blind. Personally I would never build a blind on that boat. Kinda defeats the purpose of a boat blind and hunting comfortably out of a boat. If you go with a bigger boat get a beavertail blind and put blindgrass material on it. Expensive but it should last a lifetime since it won’t rot. Make sure to cover the shooter holes. Every Tom, dick and hairy has a boat blind and almost no one covers the holes in the northern states. Ducks figure it out quick.
I prefer not to hunt out of a boat blind. I have a half dozen duck boats ranging from small to big including a big Lund Alaskan with a blind. We hunt out of the boat with a blind maybe 5% of the time. Only do it with locations that require it. If you can walk on land or in shallow water in waders it’s best to put the boat down shore and hide it away from where your hide is. I see so many hunters that will park a big ugly boat blind in front of great natural cover. Sure sign of a rookie.