For smaller engines or those with a cast in backhead I just use a 2x4 with some tape or paper over the end to make it snug. If the cab or backhead comes off I have length of aluminum pipe pushed in the cab end. I use duct tape around the pipe with the first wrap applied to the pipe in the normal way then back on itself with the sticky side out. That is usually pretty secure so I can turn it every which way to get all the nooks and crannies.
Pete
