When installing perspex windows on any boat the issue is expansion/ contraction. As previous post mentions The perspex expands and contracts with temp change more than the surface you are sticking it to so over time the bond to the perspex breaks.

The only product i would use to bond perspex or glass windows is 'dow corning 795' silicone. It has a 50% expansion modulus. In comparison sika 295 and others only has an expansion modulus of 25%. 

In real life application you need a thick silicone layer in excess of 6mm to handle the expansion/contraction of the 2 surfaces using products such as sika 295.

With dow corning 795 you still need min of 2-3mm thickness of silicone to stop the bond failing over time. Also the more surface area you have to glue the 2 surfaces together helps. Min of 25mm wide strip.

Also dow corning is uv stable where as sika 295 and others aren't and so they break down and lose elasticity as uv gets through the perspex.

Using fasteners every now and then around the window can help make install easier but every screw/fastener could have the potential to develop a leak so the least amount of fasteners the better. with fasteners you also use a 3mm thick single sided foam tape which gives you a uniform thickness of silicone right round the window.

You can install with no fasteners but it is a little trickier and you use a 3mm thick double sided foam glazing tape approx 6-8mm wide. you need to be pretty exact as once the window touches the boat it is stuck and if in the wrong place you will have to cut it off and start again.

The whole process can be a bit daunting but as soon as you know how it all seems to work out ok. I've installed dozens of glass and acrylic/perspex  windows and have never had one fall out.

If you want to know the whole process you can pm me or reply and I can go into more detail on a new post. 

cheers