I have needed these for years, and for years I have cobbed together whatever steel I had around around the shop to do two things:
1) Provide a straight edge for plasma cutting that was fast to set up
2) Provide the proper offset above the tip to avoid blow back and have the consumables last longer. Please note the Miller Spectrum torch I have is not really a drag type and it should be spaced about 0.062 off the cutting surface.
I also wanted to use these without the use of clamps to get in the way. The solution was those wonderful Neodymium magnets. I think the photos are self explanatory if there are any questions please let me know. The magnets are a press fit approx. 0.750 dia. so an end mill works great. The hole is bored a few thou deeper than the holder. I have made various lengths up to about 52" to cut sheets in half.
I hope this might give you gents a few ideas for other applications.