Coat the inside surface of the barbecue grill lid with strips of adhesive thermal insulating tape. Overlap each strip of tape by 1 inch over the last for full coverage.
Measure all the exterior panels of the barbecue grill that can be covered. Draw shapes of the barbecue panels, with dimensions that are 1 inch larger in every direction than the panels, onto a thermal insulating blanket. Cut out the shapes.
Assemble the panel sections so the seams are overlapping by a 1/2 inch. Use the thermal tape to secure the overlapped seams from both sides. Cut exhaust or vent holes to line up with any that you do not want blocked.
Slide the custom-fitted thermal cover onto the barbecue once the grill has reached a desirable temperature. Lift the cover off and set it aside when you are tending to the food.