I don’t have any holder for it, normally I will try to used what I have to organizer the things… 
like the photo I’ve been post in my previous topic Can’t waiting for unpacking his Marble Run🤪, I will tidy up the boards & package from A to Z and make it stand in the box which I place on top, and the bottom one temporary as a trash 
That’s a good idea for those who really need it, if you can add some drawer or parts at the bottom and it will be more useable 