I was joking all weekend with folks "isn't it just terrible how we force the dogs to do this?" as dogs were leaping, screaming, dragging us along to the start/finish line, chewing lines because they couldn't go soon enough, etc etc etc. Even Rogan after coming around the race course started jaw-jacking again when I took Dulcie out for her 'victory lap' afterwards.
To anyone who thinks it's cruel: they've invited to hang onto a tie-off line and see how long they can hold onto it when I yelp 'avacado' or 'Gertrude' to my hooked-up team waiting to go.... it doesn't matter WHAT you tell the dogs at that point, they're GOING, and rotsa ruck to the skin on your palms and your shoulder sockets- lol!!!
If nothing else you can always hook the dog up to a tire to drag on 6' or so of rope as you take him/her for a walk. Cleans up a tire from out of the ditch and burns energy out of that dog VERY fast- in fact, you might want to start with just 5 minutes on the tire and watch feet carefully as they'll wear quickly.
If you've got the equipment and know-how to create a scooter yourself, I'd say go for it! And you may still want to join the yahoo group Dogslovetorun and put a post on there re build-it-yourself... hopefully you'll get a response from our very own Les, who has done a few I believe (and I've run with his team of rescue sibes- he's a super nice guy). You may also get a shout from a couple others. I think there was a link to some australian site with plans for a couple cut-down bike frames way back when.
I've tried serveral kinds of harnesses and X backs have always worked the best for me... they are the easiest to fit correctly and if you just can't bring yourself to order one, making them with some dental floss for thread isn't hopeless. Although you'll honestly not be that much ahead of just ordering one for the price. If backing out is a concern, just make a tie out of a rag or use a neckline or something to put a belly-band on the X back and you'll be set. If you need a picture of this contrabulation let me know.
Having the dog start out pulling a drag is a good start, then keep encouraging him to get out in front of you while pulling at the same time. I have used a flexi leash while doing this kind of training because I can end up a ways behind running (gasping!) behind the dog and the tire. If you're headed towards something the dog REALLY likes to get to- a pond or home or the car, whatever- it will make your efforts that much easier.
I like using a car tire as a drag but you can use a chunk of wood etc. Just beware of the dog's reaction to noise and that the dogs will wear out rope grinding on the ground VERY quickly, which is why I prefer to have a rope loop through some holes in the tread of the tire. The other nice thing about a small car tire is that it doesn't bounce too badly- one less thing to worry the dog with when starting out.
And the other good thing about hitting that scooter list is that there may be other folks with trained dogs reasonably close by. The fastest way to train a harness sport dog, bar none, is to hook it up next to a seasoned dog. We humans are very clever but we still speak doglish as a second language. Dogs can tell each other about this stuff VERY quickly.