The 'heater' design evolved from the flat-top kite shield in the late twelfth century. The name was given by the Victorians after the irons they resembled.
This design of shield became popular among the elites of Western Europe in the very late twelfth and thirteenth century. It should be restricted to knights as it was essentially an item of cavalry equipment, though it may be an option for well-equipped Serjeants in the mid-thirteenth century. In the thirteenth century, shields seem to have been much less common in the hands of lower-status infantry, who were instead armed with pole arms and (increasingly) bows.
Heater shields are made in the same way as kite shields, but should be somewhat smaller. They should be designed as a square with (roughly) a triangle added on and should be wider than the warrior’s forearm. They should be made of wood and covered in linen or fabric. Heater shields should not have bosses. Heater shields should be painted in the coat of arms and heraldry of the knight.
Images of heater shields
The surviving arms of Edward of Woodstock ('the Black Prince', 1330-76), preserved in Canterbury Cathedral, including a heater shield.