A ready-made trap cube that erupts 50-cm spikes in a one-meter square — mountable even on a door.
The Spike Module is the second dice-sized trap cube from Princess-Donut's second-floor gold sapper's box. When triggered it pops 50-centimeter spikes out of a one-meter-square area every two seconds — and once triggered, keeps spiking forever, so it must be placed carefully (friendly crawlers and NPCs don't set it off). Carl discovers it can be affixed vertically to a door, mounting it on the Jikininki-car entry so that breaching Drek impale themselves, buying the party time and a trickle of experience during the train siege.
A tiny dice-sized cube that deploys a blinking one-meter trap-square erupting with half-meter spikes.
A door-mountable spike trap that shreds mobs trying to breach the party's train car.