Steve,
I had exactly the same issue when making laser collimator for the spectroscope. I found one solution was to prick a VERY small hole in quite thick aluminium foil and wrap this around the front "nose" of the laser.
Tightens up the beam and gives a pretty circular image.
See:
http://www.iceinspace.com.au/index.p...63,500,0,0,1,0