If you're going to replace fixed plexiglass windows, I have heard of two methods: one uses a special double stick foam tape to affix the window, and then Dow 795 is squeezed into the remaining crack. But I have also heard that you can seal and adhere the window without fasteners using only the Dow 795.
If you do this, I saw someone's replacement photos, and they took 2 or 3 blocks of wood and used double stick foamy tape alongside the windows before removing them, so they had guides to put the window right into place.
Brian