Just do what an Ork would. Get a few choice weapons/bits/rocks/etc and mash them together. If any one asks what it is just say "conversion beamer". If they question it further or kick up a stink then give them a good seeing to with something heavy.
Since there is no set look for the
CB then it can look like anything. If i were to make one i'd start with a man portable lascannon and heavily modify it.
Also have you tried looking at 3rd party bitz companies? You might find something that you like the look of on Maxmini or Hasslefree or maybe even something from Anvil Industries.