Well the simple answer (as has been done countless times before) is, in basic terms, to mount a board onto a couple of spring clips
then you use the spring clips to clip round your strut brace, the board dangles below and he presto keeps the rain offf your engine top.
Of course for true racing pedigree you would probably use a sheet of stock carbon fibre with a sticker on it. But if you used a bit of hardboard or your mum's tin tea tray you'd be doing just as good a job.