I came up with a nice solution for a projector window that solved a couple of problems.
Problems
(1) If the galvos are too far back, the hole and window need to be larger.
(2) How to clean and protect the window?
Solution
Since my galvos are a bit back from the edge, I require a 40mm x 40mm exit hole. At first I looked for a piece of glass larger than that but then I reliazed it isn't required. If I put the glass closer to the galvos, I can use a smaller piece of glass. So, my scanner box has a 40mm x 40mm exit hole. (BTW, why do people make round exit holes on scanners when the laser outline is square). Then I built a square tube out of aluminum pieces. That is 40mm x 40mm in diameter. It is only about 1" long so it really isn't a tube. More like a cookie cutter. On one end I mounted a flat piece of aluminum with a slightly smaller hole cut through it with a hole saw. Then I went online and ordered a camera adapter ring and a HOYA filter. I epoxied the adapter ring to hole on my cookie cutter tube and then screwed the HOYA filter onto it. The cookie cutter tube is mounted inside my scanner box at the exit hole.
So now, I have the HOYA filter about 1/2" from my galvo mirrors so I can get by with a normal camera filter with no problem. Also, since the tube sets the glass well back from the front of the scanner, the reduce of accidently brushing up against it is very small. And, if it needs cleaning or replacement I can just unscrew it and stick another one on.
Next I am going to machine a nice door for the hole at the front so that I can open and close it. I have to machine my own hinge though since I want it to have a very tight fit when closed.