I'd start with buying an inexpensive bimini online with at least 3 bows. That's what I did. Then I went to a canvas shop to have them make up some front and side windows.
They sewed zippers in the back ends of the bimini and windows in case I wanted to extend out a camper top or add a storm curtain later on. Mine came to about $900 with all Sunbrella fabric.
You could make sides with mostly fabric where I have all windows and cut in what ever size windows you want. Sew zippers onto all back edges.
So if you were doing it yourself if you start with a store bought bimini with a cheaper fabric the support bows are pretty much all the same aluminum poles unless you are in salt water and then you need to use stainless which costs 3 times as much.
But if you start with the bimini you will be half way done for around $250. Pick an off color and get it even cheaper on sale. Or buy a used one off ebay or craigslist.
Then for the camper part just add the back part of fabric and the sides with a few plastic windows sewn in and the support poles. You will need to use a special sewing machine to punch through the canvas and use some type of marine thread that does not break down when exposed to sunlight.
I only have a foot or two exposed at the back now where I fish off of. And I used a standard bimini to start with.
The grey thing in the window is a hanging enclosure that I clip on for a changing room or when using the porta potty when other boats are around. I paid $60 on iboats ready made up. But it's just a piece of canvas with a zipper and four grommets at the top.
Wifey loves that accessory.