After fighting with blankets falling off, I decided I really must do something about this.
The commercially available ones are all a bit pricey, so was looking around to see what I could use. It is a bit "left field" but looking at a table protector we had on the dining table, it looked ideal.
It is a 4mm mat (felt topped by a vinyl / leatherette covering). It is readily available in a choice of colours, inexpensive and easily obtainable.
I bought a bit 140cm wide and 2.5m long, cut out a hole for the bonnet and ...... it works great.