I have tried two types of harness.
The comfort harness - this is a picture of one on ebay
I personally did not like this for my bunnies because for some reason they could slip out of it, and I had a harder time securing. Maybe it was just me.
The 2nd kind, I got was the Deluxe harness - it looks like the same kind for cats and dogs, and you could probably use ones made for cats, but just be sure the bands are as wide as this one. Rabbits will go where they want, and the thicker bands are more comfortable, and won't cut, rub, or burn. Anyway, this one they did fine on, but there is some technique in getting them used to it.
Here are some more options and reviews There are tips there too about how to loose or tight it should be.
Whatever you do, don't put it on and go with the leash as some will freak out, because it may make them feel as they are caught, and they can seriously injure themselves.
The approach I took worked well for my bunnies. Some bunnies may never take to it. But this is what I did.
The first couple of days, I let them sniff the harness, I petted them with it, basically let them just get used to its presence.
Then the next few days, I put just the harness on them. (short sessions once a day until they got used to it) Bailey didn't even notice it was on, (she was just glad the ordeal of putting it on was over - but that got easier with time), and Rucy didn't like it and tried to get it off, but after a few minutes, she forgot it was on.
Once I could put it on them, and they no longer cared, I then added the leash element to it. Tip - make the leash long enough that he can go on his merry way exploring (in the beginning), and you can just very gently tug. VERY GENTLY - this is when you will find out if your bunny is okay, or will need more time to get used to it. (of course do this inside or at least in an enclosed area, so you can be sure the harness if fitted appropriately.)
As my bunnies got used to the gently tug, and didn't freak out, then I could exercise a little more control with the leash. But even then, I never really controlled. It was more of a coercion. I learned their behavior and how to work the leash with their personality.
If you take your bunnies to a new outside area, just be sure it's pesticide free. And of course, you can never be sure there are not any parasites. So when I a bunny is allowed outside, you'll need to keep a close eye on his poops, and his health.
There are dangers with any harness because if a bunny is startled, and bolts, and runs full blast with tons of lead, once he hits the end, he can flip up and around and cause serious injury - like a broken back.
I ended up choosing to keep the leash no longer than 10 feet, and keeping close by, (just letting him walk me) and if I felt he was getting nervous, or if I saw his head bolt up, I would, keep the leash somewhat taught, to prevent that bolt. He might tug against it or try to wriggle out. But it prevented him from a full bolt.
I can tell you that my bunnies loved exploring this way. Not all bunnies will and of course, you just have to be careful. I actually just saw a book on the net about how to use a bunny harness, but now I can’t find it!
I haven't used any harness lately because we moved, and we don't have a safe outdoor place for them to be. Plus, I never did use one on Jack. He's such a nervous big bunny, that I'd be too nervous.
Let us know how it goes.