I run a rope from the bow to the stern, you can usually use the cleats,
and I have a 2x4 that I cut a small notch in to hold the rope and prop
it up as tight as it will go right in the middle of the boat, or as
close to, as you can get. I have a old rode that I use for this. Put
your tarp over it and secure it under the trailer/boat with smaller
rope or bungies.

All this aside the best thing you can do is visit your snow covered
jewel once a month with a push broom and sweep her off. I find that
doing this during a thaw is especially important because the melting
snow is very heavy and will turn into ice when it gets cold again.

A thought about tarps: I have a oversized one and with the folding
tounge I am able to cocoon the entire boat right down to the tires. It
will never make the Victoria Secret Boat cover catalog :-) but it does
keep everything well protected and dry.

Good luck and happy snow shoeing!

Randy