The last time I did the trimming around my yard, and around the 4,700 shrubs, posts, and planters in it (okay, that's an exaggeration, but you get my point), I ended the day with an aching back and a sore hip, and I swore I wasn't going to lug my trimmer around and use it the same way ever again.
So this is what I came up with. It can be attached to any trimmer in the same way you see here. I hope it saves your back too.
In a few days, I'll post a short video of it in action. By all means, use the comments for any questions.
The "tee" that's strapped to the trimmer shaft has been cut to only a half-round, and the elbow fitting below it is angled at 45 degrees, not 90. I'm still refining it, but the general concept works well. I didn't even glue the PVC connections together, yet they're still sturdy enough with just a friction fit.
There's a 3/8" threaded rod through the bottom tee and its flat-top end caps. The end caps are drilled with a 3/8" hole in their centers. The 3/8" rod has a washer and nut at each end, which keeps the end caps tight against the tee. PVC spacers, which fit over the end-cap nuts, are next, followed by a washer and then the wheels. And finally, a pair of nylon-insert lock nuts over washers keep the wheels on the rod, Those outermost nyloc nuts aren't tightened too much, so as to let the wheels spin freely.