This set of holders is for holding the IKEA Lots mirror in place while providing clearance for a 4mm-5mm cork insulation under the bed. The cork insulation ensures that the bed temperature remains even across the bed, and also maintains heat for a longer time in case you want to start another print right away. The cork sheet needs to be attached to the underside of the bed using temperature resistant tape or glue.
Align the parts as pictured since, if it breaks, it will break along the horizontal plane of the part. The prints are usually the weakest along the layer lines. Hence I chose that alignment.
It's better to use ABS or PC-ABS blend since the bed temperature can easily reach PETG or PLA glass transition temperatures.