I only use Medium Flat rate. Large are just too big and heavy but I have done it.
All of them are done the same. I have access to a box maker so I can make the cardboard inserts to my exact specs but you can just use a paper cutter or whatever.
For 1 Medium Flat Rate box you need:
3x Sheets of cardboard (bottom,middle,top) 11x8.5"
6-8x Sheets of cardboard (dividers) 8.5x2.5"
1. Sheet on bottom
2. Divider,cards,divider,cards,divider
3. Sheet in middle
4. Divider,cards,divider,cards,divider
5. Sheet on top.
6. Tape all edges and sides where box is glued together.
I have had near 100% success rate this way. Only one damaged and that was the very first one because I did not tape the edge where the box comes together.
You can get roughly 3K regular cards and 2K chrome cards. 3 Rows on bottom, 3 rows on top.
You can do all this with the large as well, you'll just need to adjust the sizes.