This is for the Monoprice mini delta. The cooling fan blows directly on the the back of the heatbed where the thermistor is and this is intended to plug the hole. There are two prototypes, one with ears for magnets to be attached under the bed and one with no magnets to fit under the bed. The magnetic version is a little too thick to fit under the bed like in the picture but it gets the idea across. I have not printed or tested the non-magnetic version.

Since this is near the actual part that heats up on the bed, be sure to use a high temperature material like ABS or preferably PETG. I originally tried to block the hole with kapton tape but the heat caused it to rip itself apart. Others have used a flash card taped to the inside of the printer. I also suggest using concentric supports for the lip with about 25-30% support density (the lip is small so a lot of support is needed).