Sorry for this REALLY late answer!
But back to youre Question, It's easy 1. make 2 boxxexs the 1. need to Collision groups of the Shells then you must make it so that you have a box where you can drop them in. The second box dont need COllision groups now water no a,b,c,d,e,f, or anything then select the 2. box and click on the button for "set in forground", then combine them so that you have a Box i hope you can understand this. cause my English is really bad when i need to Explain anything. But Thanks for this Question!