Dropped in at Lidls the other day to buy one of the met stations Dave Croft mentioned. They didn't have them in, so I browsed, naturally.
I found some decent, solid, woven plastic shaped covers intended for garden furniture. The one intended for a stack of garden chairs (£1.29) would nicely cover a vertical engine such as a Lister A or even an L. A larger one (£3.29) is big enough to cover my Honda XBR to the ground. They come with big eyelets around the reinforced bottom edge but no cord.
There was also a range of fold up chairs that looked sturdy enough to entrust with my not inconsiderable mass, but I have a fishing chair I inherited from a widow that I am concerned to find cost a small fortune, so I am enchaired for the moment.