This allows you to insulate the bed of the printer so that it heats to temperature faster.  When the printer starts, the insulation is pushed off to the front.  I have a video of it, but this site does not accept mp4s so I zipped it.  The pictures included are of a sheet of Harborfreight flooring used for insulation (easily handles 110 deg C; does not handle extruder temp of 170 deg C.  The brown plastic square that is glued (epoxy) onto the insulation was 3d printed in pla. The warmer clamps are used to hold the box in place and extend through the foam.  They are printed in ABS to improve temperature stability which may or may not be necessary.  This allows my bed to get to 110 deg C in 18 minutes rather than 45 minutes, and I don't have to be there to remove the insulation. 

It needs to be tall enough to not allow the extruder to go over it.   The 3d print is basically a box where one has turned on support.  While a bit fragile, the support acts as walls for the box.  The height needed for the box is determined by how high the extruder is when you start the print. If too high the extruder will clear the box and then try to crush it.  I have posted the video at https://youtu.be/MFVoMciOrhQ