Getting back to Nick's problem - could you mount it on a small washboard, possibly one specially made for the purpose instead of one of the regular keepers-out-of-the-rain. You'd need to be able to reverse the washboard so the plotter shows either in the cabin or outside. You'd need to step over the washboard going in/out or else remoe it each time you go in/out.
Could work on some boats I'm sure - whether a Vega has suitable arrangements I don't know.