I find downloading someone else’s design and then reverse engineering it to be the best way to figure out how something works…if your brain works in similar ways I’ll point you at: Deck Box for sleeved cards - glueless and Duel Deck Box (MTG) (do a search for “deck box” and you’ll find more)
Also there is @kigster’s online box generator at https://makeabox.io/ or if you use inkscape this leads to a plugin: Box Generator
Have fun!