UPDATE: Added model for 4 plugs too! This model uses 8x2mm magnets instead.
Case for three spark plugs with 14mm thread and 21mm HEX, which are found usually in dirt bikes, go-karts and other similar kind of vehicles and engines.
Designed for four 10x2mm magnets which are keeping the two parts together nicely, sure it kinda works without them too or using something like rubber bands etc.