You've got a couple of options. *You can try what skools said or a drift sock. *Both work but there's storage and deployment issues. *Forgetting to pull the bucket(s) in could result in damage to the boat or even you or a passenger.
I think the #1 way to slow down (if you don't need to sneak) is with a kicker motor. *I have a 9.9hp on a bracket and it works excellent. *Can get down to 1mph if I want to or anywhere between 1 and 2.
Now if you wanna sneak - get a nice bow mounted electric trolling motor.
Then there's trolling plates. *I've had experience with the one below on my old 17ft boat. *Its called a Hustle trolling plate. *I loved the plate. *With the springs on it you don't have to worry about engaging/disengaging anything. *Boat worked well in reverse too. *I could go as slow as I wanted using it too. *The downsides are - 1) having to drill holes in your cavitation plate 2) Its permanently mounted and alot of people ask "what's that?" 3) The springs and i-bolts are not stainless. *You can replace the i-bolts with stainless but I haven't found stainless springs to fit and they would be expensive.
Mind you I've never tried it on a V20 but I'd be willing to bet it would work great! *If I didn't have a kicker I might go that route because I like to troll flounder and trout in the creeks.
http://www.johnsonmarinesupply.com/h...ing_plates.htm
Also the guy at johnson marine supply is top notch and will be happy to speak with anyone over the telephone that might have questions. *I've called him a couple of times *;)
You might also want to look into some of the other brands of trolling plates, but I looked at a lot of them and I really like the Hustle for ease of use and the hole allows you to go in reverse.