Warning!
While the chinese CNC6040 from carving-cnc.com has DIN 508 compliant M6x8 T-slots,
the CNC3040 from the same vendor has something smaller.

It's way too large for M5x6 and ever so slightly too small for M6x8.
http://www.schrauben-lexikon.de/norm/DIN_508.asp
a should be 8mm with the nuts being 8-0.3=7.7mm. However it is 7.58mm wide.
The next smaller standard size would 6mm.

Since M6x8 is the smallest I can find with the usual vendors this is a problem as I can't properly fix anything to this table.