Double adhesive foam pads
I bought one of these from Ebay: [
www.ebay.co.uk]
I took it apart and sanded a little bit, so it will drop down on the print-bed. When using it, I just attach it with a double-adhesive foam pad.
It actually woks pretty well. I measured it against a high quality caliber - no deviation that will affect my measurements.
Please note, that I'm only using it for relative measurements when I check the mechanics and align the bed.
Best regards,
Carsten