I used painter's tape (the blue stuff) to tape the thermocouple to the group. I covered the tape with one of those silicone wrist bands and then wrapped the wire around the group, again around the neck of the boiler cap and then taped it to the back of the machine. See pictures below.

I have velcro tape and was going to attach it to the machine with that as the magnet in the reader is barely strong enough to hold it on as long as you don't bump into it. But I was worried that the heat from the machine might degrade the reader over time. So I only use the magnet to hold it to the machine while not in use. When I make espresso I just put it on the counter next to the machine.