So... different way to do it.
Summon the chest. Hand the tiny chest to a friend. Climb into the Leomund's chest. Cast magic jar on a jar your buddy is holding (You can hold it, too). When your body collapses into the chest, he closes it. Carries the jar into the fight. You jump outta the jar to possess a dude, take the tiny chest, and summon your Leomund's. Bam. There's your body. Right where you left it! Drag your carcass out, climb your opponent's body into the chest. Put the jar on the edge of the chest, then close the lid to break it. Your souls go back to the right places...
But your nemesis is now trapped in a Leomund's Secret Chest!