When I first started towing F whole touring I just tied a rope around her waist and went for it. This worked ok for short distances, but it was clearly a lot of work and uncomfortable for her. So (at 2am the night before a trip, of course) I decided to make something better. It's been through a few tweaks, but at this point I think I the overall design is pretty good so I'd like to share it. Key design features:
To make it you'll need:
Finish the sides of the fabric seat first. You will eventually want the bungee inside the fold you make when finishing the top and bottom of the fabric seat, but the rest of the steps will be a pain with it in there. For now just sew a piece of thread into it instead; later you will use it to pull the bungee through. I sized my fold so it is a snug fit for a double-fisherman knot with the bungee I used. This let me wiggle it into the middle of the bottom, where it is out of the way, and it stays there.
Sew a short piece of webbing to each side near the top for the waist belt. Thread the waist buckle through these so it can be used to keep the harness tight around the waist, like a belt. I sewed mine back a bit from the edges, so it doesn't get pulled loose when the harness is under tension. If your harness is wider than you need you might want to sew it pretty far back.
Sew the two suspender straps to the top edges near the front, and sew the snap-lock buckle to the center of the back. I tied my suspender straps together so they join right in-between my daughter's shoulders, then passed the two of them together through the snap-lock. On-trip adjustability for clothes using just the snap-lock and waist buckle is adequate, and I can move the knot at home beforehand as she grows.
Now thread the bungee through, tie it in a loop (I used a double-fisherman) and arrange the knot so it's out of the way (I put mine in the middle at the bottom).
You're done! Now you can clip the bungee loops for towing uphill, and for downhill get your kid to ski over the rope with one ski so it ends up folding back under their crotch. My daughter claims this is comfortable, and we do it a lot so I believe her. I use an old cordellette for my tow rope and find it to be a convenient length.
I've thought about modifying the design a bit more to put in a crotch-loop, so it can't ride up, but it doesn't seem to actually be a problem for us so I haven't bothered. I've also thought about making the bungee even longer so I can wrap it right around each side to clip on the downhill, but my daughter claims the current method works fine, and it is convenient to be able to switch to the downhill by flicking the rope forward and having her ski over it. Here's F demonstrating the transition to "downhill mode"