Summary

Here's a bike rack to suit my daughters 24" mountain-bike.

I used 1/4" steel rods cut to length and glued with Araldite in the following order:

1. Verticals into the branch;
2. Diagonals into the branch;
3. Front, Back and Middle onto the verticals / diagonals;
4. Horizontals (inserted through the holes at the back.

The SCAD file is all parametric, so adjusting it for other sizes shouldn't be too tricky.

Now we're off for a week long holiday with lots of riding. I'll report back at the end on whether it survived :)

Well, we made it back in one piece and so did the rack; it even acquired a little basket along the way. Daughter 2 is pretty keen for one now.