It amazes me that people pay such a lot of money for these things, but I guess that’s the fly fishing industry business model. Anyway, don’t buy one, because it won’t be right for what you need. If you want it to be right for your boat and the conditions you want to fish in, you’ll have to make it.
Good news is that it takes about 15 minutes to make an excellent drogue. Just get a square of fabric (waterproof nylon is good, but an old bedsheet did the job for me for about 10 years). For a pontoon, you probably want something about 2’ - 2’6” square (a fair bit smaller than the commercial ones, which are designed for using with a big rowboat in a big wind). Fold the fabric diagonally twice, and cut off the point to leave a hole about 1 1/2” square. Get some parachute cord and four small pebbles or split shot. Cut two lengths of cord about twice the width of your fabric. Bundle the pebbles into the corners of the fabric, and tie the cord around them. The pebbles stop the cord from slipping off, and add enough weight that the drogue deploys quickly. Using a couple of big split shot on one side and a couple of corkies or something else that floats on the other might improve that even more, and help reduce the potential for spin. Tie an overhand loop in the center of each of the cords, tie another length of cord to those loops and you are done.
I used to carry three different sizes of drogues, because I was fishing competitions and sometimes had to fish in pretty windy conditions, but the smallest one got the most use. I much preferred square drogues over conical ones, because pulling on one corner of a square collapses the drogue and gets it out of the water. Conical ones are harder to deal with. The paradrogue style with a cord going to each side are sort of steerable (by changing the length of each side), but in a rowboat, changing where the drogue is attached works better for changing the direction of the drift. I doubt that either way will do much in a pontoon, but you have other ways to steer. You can change drift speed a bit by choking the drogue down - just tie a piece of cord around the 4 corner lines and slide it up or down to change how much the drogue is able to open.