This may sound like a second rate solution, but I have added a red laser to my build for around 5$ by knife-edging the second beam next to the first.
Instead of using a beam combiner to directly place the red beam inside the cutting beam, you can carefully place a mirror right next to the beam and bounce the red laser so that it is right next to the original beam. This approach is very common in cheaper laser projectors. You can see an example here:
http://laserpointerforums.com/f50/octup ... 60584.htmlEight seperate beams are being grouped together by carefully placed mirrors. As you can imagine, you are not going to get a "tight" or accurate beam, however after going through the focusing lens it doesn't really matter.
Believe it or not, I aligned my red dot using only sticky tac. Since any beam hitting the focusing lens will be directed to the focal point, you have about half a degree of play to aim the laser. It took a lot of fiddling to get everything just right, but it took me only around an hour for the entire thing.
Accuracy is not a problem, at the focal point the red dot is aligned better than I can see which is all that really matters. I can get pictures later tonight.