One way to do it is select a block after your cut off - ie if at closing we are block 690250 - then maybe you pick block 690255 - if there is no winner at that time, you go with the next block(s) until a winner is found.
Edit - after reading your post above, I think that might be what you are planning.
Yes, that is another way of saying what I am planning.
I will pick the
1st block after the cutoff time, and if there is no winner, I will pick the next block, and will continue picking the next block until there is a winner.
My goal is to both ensure there is actually a winner and a second-place winner and to do so in a way that is fair and that there are no accusations of any kind of shady back-room dealings, or anything of the sort.