This is my version of the mantis router, http://makeyourbot.org/mantis9-1 as designed originally by David Carr. This design is adapted to metric sizes (do away with Henry IV's thumb!)and enlarged to a groundplate of 30x30 cm. The working volume is slightly more than 190 x 130 x 40 mm, depending on how precise you work.

I used a larger CNC router to cut out the wood pieces, I suggest you do the same. The material used is 12mm birch ply (that's ~1/2"), but any reasonably straight plywood should do.
I designed in Rhino3d (for want of something equal but open source on linux...)so the rhino file should be read as authorative, the dxf file is there for your convenience. There are several layers in there prefixed with a number, Follow the instructive layer names in that order. An offset for 3mm drill and 5mm router bits is supplied, but you can of course generate your own...

further (probably incomplete or outdated) instructions here: http://wiki.protospace.nl/index.php/MantisRouter.

[EDIT]: I updated the plans to use a more common 'fake dremel' and added lots of extra tweaks. The nesting as it is now is tailored to a 5mm flat tip bit, with unneeded (double) cutting paths deleted. Those of you in the US might have to generate a new cutting path to accommodate an imperial size bit. [/EDIT]