My M6 tap ("WALTER METRIC 6.0MM TAPER HS TAP - 03-D061") from HD Supply came without a case, so I lasercut one.

If you need a handle for your tap see http://www.thingiverse.com/thing:5672 - at some point I may try to merge these into one object.

Lasercut in 3 mm acrylic. Use tap to cut M6 threads into the end holes in the one layer that has the smallest holes (left hand side of pdf file). Stack layers and bolt together with two M6 bolts.