The parts you require:
1 x Hose clamp, the screw type.
1 x 4mm x 40mm tapered head machine screw and nylock nut.
1 x 3/4" electric conduit joint/coupling. Make sure to get one with the shoulder on the inside as this will be used to keep it in place with a washer/spacer
1 x washer/spacer that fits snuggly but does not fall right through your conduit joint, with a 4mm - 5mm hole.

If you cannot find a conduit joint of this type you can make your own by glueing a small ring of a smaller section of conduit on the inside another bigger section using PVC glue or superglue. All in all this section is only 50mm - 60mm wide and 20mm - 25mm in diameter to take the standard mounting bracket from the speedometer.

Drill a 5mm hole through the ring of the hose clamp. Grind the one side of the conduit joint so it sits snuggly against the pipeclamp once it's mounted on the frame.

I start by mounting the hose clamp with the screw in place on the fork above the footpeg and clear of where my feet might catch it. It's a good idea to have the screw unit of the clamp pointing forwards and out of the way. Then it's just a matter of fastening the coupling in place.

This is how you put it all together.

You can grind two extra notches in the coupling to "hide" the extra wire from the speedometer's sensor by wrapping it around the machine screw. The wire enters at one notch, goes around the machine screw a couple of times and exits through the other. Just make sure you don't "pinch" the wire. I also protect the wire by running it through a heavy plastic sleeve cut open lenghtwise and taped up again with electric tape after the wire has been inserted. A good source for magnets are old headphones as magnets supplied with the speedometers aren't always practical for our intended purpose. I've found the old walkman style earphones have a magnet of about 10mm diameter that can be removed quite easily. They can be simply glued to the rim of a wheel.

Modify at will!!!