I remember in an old issue of Photo Technique magazine, they had converted an old Omega to use the then new CFL type bulbs. It worked despite the long start delay. There's no reason really for the enlarger not to work using an LED bulb. The only reason why enlarger bulbs are so expensive is that they have a very even surface and no visible logo on the top. If you can find an LED bulb of similar output, it will work.
As for the negative holder, cardboard can work fine, as long as it's relatively rigid. It really depends on how the enlarger holds the negative holder. Two of mine use simple leaf springs to put pressure on the steel holder. But my Durst is more sensitive in that area since it relies on the entire enlarger to hold it down (it's a strange design). Besseler use a large lever system so theirs is a bit harder to imitate.
One thing you could do is make one out of bent cardboard to try it out, measure it, draw it using a vector based drawing program like Inkscape or Qcad (they're free) and send the file over to Ponoko to have it laser cut. This would be the ultimate in quality.
