As result of the work of @m26872 and @GertSanders supported by this community my "need" for a 3D printer is made redundant. The minimal boards like the My Slim 2AA Battery Node and GertSanders "One trick pony" make a housing superfluous.
Some minimal examples here on OpenHardware with 'real' hardware:
 
Standard (3/4 - 19mm) electricity pipe. Also fits a CR123 (3v) battery nicely.

Impact resistant (5/8 16mm) pipe (sensor powered by 2AAA/ or 2 'hearing aid' batteries.
An all nature housing in bamboo.
And the ultimate one, no housing. Hidden in the door and door post.
You can cover it with tape and/or paint
all the tools needed:
| Qty | Value | Device | Package | Parts | Description | MF | MPN | Aliexpress or ebay link |
|---|---|---|---|---|---|---|---|---|
| 1 | pcs | Very narrow and minimal switch node | custom | http://forum.mysensors.org/topic/3086/very-narrow-and-minimal-switch-node | ||||
| 1 | pcs | Door contact/ reed switch | http://nl.aliexpress.com/item/5pcs-Recessed-Magnetic-Window-Door-Contacts-Alarm-Security-Reed-Switch/2006368350.html?isOrigTitle=true |