Thanks CA, but at the time of posting the Ender 3 V2 seemed to be printing OK. My worry was because I expected trouble because of there being so little material on the base plate. But not sure enough to stop.

My later edit showed that I was right. (Which, of course, now seems obvious!)

BTW, is it unrealistic to expect the author to position the model (in this case the top of his box) so that the large top surface was on the base, so that the user doesn't have to do so? I finally learnt how to do it in Cura and right now I'm 50 mins into my second attempt to print it.

For future reference, it's Friday 2 April 2021, 1816 here; what's your local time please?