I have a 26' Seafarer, which is not all that much shorter/smaller than your 303 and I use a 25'x30' tarp over the mast. The mast is supported at the bow pulpit at the mast step and at the crutch which is at the wheel. The tarp cost me $75. I either take down the stantions and lifelines or leave them up (depends on if I plan to work on the foredeck or cabin top during the winter) with empty plastic cups over the tops of the stantions. The fore part of the tarp gets tied shut, the sides get tied together under the hull and the stern gets woven together to allow me to get in/out.
Used this method for 3 years w/o any sign of damage to the mast or other bits of the boat. 3 years is about all I get out of a tarp though due to wind/weather/ice/snow, etc. Without the stantions in there generally is no issue with snow staying on the tarp, too much of an angle and it slides right off.
I've heard many people say that you shouldn't use the mast as the center support for a tarp like this but in my experience, so long as it's properly supported, there's no harm in it. Takes me about an hour to put the tarp on and get it tied down so it stays through the winter. I do have an advantage though in that I keep it in my yard next to the garage