when I want to slow down speed, I just hit the - button on my I-pilot remote. Look at a good trolling motor. work great!!
Another vote for trolling bags. One off each side, attached to front cleat. Then another rope securing them to rear cleat so they don't drift out to the side. Simple as can be and out of the way about midship. You want them to go just a few inches under water on the fronts, then tie tear rope and wrap around cleat to take up slack until their straight and kept along boat. I've set up a few boats with them, and could help you on the water this weekend if you want. Last ones I got were pretty heavy duty from north woods and only $40. Get a size bigger than your boat calls for. When I drop my boat in gear, I idle at 1.3-1.5 with a 4.3 I.O...with no bags, 3.2 with a 21 pitch prop. A smaller prop would help also, I need a 19. If you too big of bags, your boat will burn more fuel working harder to get speed up to trolling speed also.