I bought an elf when the Elf on a Shelf craze got going.
Then I couldn't be bothered with all the rigmarole of moving him every single day.
So for the past 7 years DS's elf brings him a onesie, a small Christmas type book or new PJs then pops off to help Father Christmas with his deliveries. DS would give him a hug, have a chat (no parents allowed but we would stand looking through the door as he had this very animated conversation with his Elf), then he'd go off for his bath and the Elf would 'disappear' leaving 'his' gift for DS. DS would have got a onesie or I'd have bought a new Christmas eve book to read anyway so why not from the Elf? The Elf would leave him a note telling him he'd be back on Christmas day and there'd be a little bit wondering what he'd be doing this year for Father Christmas.
On Christmas day the 'big' gift would arrive later in the day along with a very tired Elf who would hide with the gift. So we had a treasure hunt as Father Christmas would leave a note saying 'thanks for being such a good boy, oh and the Elf is back - do you want to play a game to find him?' with little clues like 'I'm safely tucked up where you go at night' - the elf would be on the bed with the gift.
The Elf stays in DS's bedroom during the year - often tucked up in bed with him. He's a much loved toy.
DS obviously knows the truth about Christmas now but the past two Christmases we've had to do exactly the same thing. It's part of DS's traditions now and it means a lot to him - it's the start of his Christmas.
So, no we don't do Christmas Eve boxes as such - we've just adapted our Christmas traditions to work with our DS.