Thanks mrmum / Chris Royle / Johnm. It looks like there are a few options. The York gustlock does look a little on the pricey side for what it is, particularly with the inevitable shipping / tax / customs, etc. The description on their website made me smile: "...the least expensive and most effective wind and theft deterrent insurance for your airplane you can buy"
Johnm - The Wind-Lock looks good, though I did have to find another website to see a picture (
Aircraft Glare-Shields) I'll see what I can rig up myself for the time being, and when I have returned from my Scottish adventure (hence the breezy parking spots!), will PM you if a purpose-built solution like this looks like a worthwhile improvement.