My copy is sleeved (Mayday Premium) and I couldn't get the cards to stay in the box without pushing up the lid, and other inserts wouldn't have helped with this. This insert fits snuggly in the box and holds the cards squished down (it's a tight hold but shouldn't damage the sleeves) so that the lid can fit on without being pushed back off.
There are two versions of the file, one with more cut out of it to save on filament, but in case you to print a simpler shape I've included a fuller framed version. Both have gaps to help you grab the insert to take it out of the box though.
My print in the photo is hashed as I print without tops/bottoms/walls to save on filament. It’s missing some of the bottom of it as part of the print failed but I couldn’t be bothered to reprint it all…