Try the zig-align (much like the mirror method mentioned earlier in this thread, except that you don't have to do as much prep work). I have used it on my enlargers with good results. The inventor/seller was not the easiest person to deal with: he demanded to know what I intended to do with it before he would sell it to me. It was not designed for use with enlargers (so I lied to obtain one), but works perfectly for that purpose. I don't recall what I paid, but it was not very expensive as I remember. Here's a URL:
You can find others via Google.