Here is my attempt, not pretty but it is cheap and it works.
Tarpaulin from Machine mart about £20.00, £5.00 worth of bungees from the pound shop. Fit to one side of the boat (trailer in my case) using the eyelets and bungees, throw it over the boat and cut off the excess using scissors and then use the clamps shown in pic 2 for the bungees (not sure what they are called about £4.00 for 4 from Force 4 Chandlery) 10 seconds to fit and tools needed and they are re-usable, never let go yet!!
Total cost about £30.00 for a 6.5m boat, been on a while now, no leaks tears or splits. Best of all it doubles as a dry work area for those wet days.