Actually, a good quality neoprene trap works well with a couple caveats...
One, the tarp needs to be arched over the roof, basically yhr tarp needs to be higher in the center that on the edges, nit hard to achieve by running 2 x 4's down the center.
Two, the tarp must be secured tauntly either with ratchet straps or ropes, not tarp straps. Tarp straps loose their tension when cold.
Corners must be protected from chafing with carpet strips
Finally, the tarp need to be checked for tightness every few weeks, especially if the unit is stored outside in an unprotected (from the wind) location.