Cheap "Roland Type" drag knife holders are readily available on ebay for small money. I ordered one without great expectations. I found that while they have their limitations, they certainly can do a job. One of the key factors for success is a solid mounting but one which allows the knife some flex in the vertical. Allted (designer of the MPCNC) came up with a very neat idea to use attachment arms between the backplate and the holder to allow this flex. Full credit to Allted is given for coming up with that excellent idea.

http://www.thingiverse.com/Allted/about
http://www.thingiverse.com/thing:1860310

I wanted a simple but effective way to mount this to my CNC. I have a 2.2kW water cooled spindle and noted that at the bottom it has a 10mm high collar with a diameter of 72.15mm. I thought that a clamp type mechanism would work well and so it has proved to be. The clamp inner diameter is 72.5mm which makes a beautiful fit to the collar of the spindle. The hole spacing is 40mm (horizontal) x 25mm (vertical). I will use this as a standard on any new attachments I come up with.

If you don't have a spindle to suit you could, of course just use the drag knife attachment and screw it directly to a flat Z axis plate.

In usage, I have found that the best results are to be had by first carefully adjusting the depth of the knife in the holder to ensure it cuts through the material to be cut and not the backing paper. This is the most tricky bit. Once you have that locked down, mount the material to be cut on a very firm base (I use a mirror) However if you do use glass be very careful as it is very easy to chip glass or the blade which could be very dangerous. Plywood is a possible base but you have to sand it very flat otherwise bumps will cause an uneven cut.

Finally one big improvement is to ensure your toolpaths include moves to orient the blade direction according to the offset of the blade. I use cambam for creating toolpaths and there is a python script for creating the orientation moves which works very well.