Are you trying to make your existing bicycle computer work when the bike is on the trainer, or add something completely separate?
You should have gotten proper rollers
I am going to try to find a way to attach a magnet to the trainer cylinder. There is some clearance issues that I will have to address. I have heard of magnetic paint, that would be a godsend. McMaster sells it for 30 USD a quart, though. I just need a few drops!