I converted a lot of my old rims (and tyres) to tubeless with "ghetto" set ups before technology meant i didn't have to. Basically, if you're running 26" wheels get a 24" tube with a valve with a removable core, slit the tube open, seat it on the rim bed, seat your tyre on top of it, fill with sealant, trim off the excess tube flapping around outside. It worked well for me on my mtbs and cx - i never tried it on a road bike. The only issue was trying to remove a tyre - everything got so glued together. All that said, since they are now widely available rim strips are probably a better, more user friendly option.