Fixing pane of glass in small opening

[Socksitis]

Hi,
we have a small opening in the porch, about 15cm by 30 cm, just a little hole that lets enough light in, I need to remove the bag and piece of wood that has blocked it for a very long time and fit the pane of glass that I have bought. There is no window frame and no room to fit a frame. I was thinking that maybe where needed I ad a lip around the outside edge to give something for the glass to butt up to and then embed in silicone?

ANY suggestions would be very much appreciated. All being well I may have added a photo of when it just had fly mesh and a plastic bag tacked on the outside

 

[Socksitis]

Not very good with the cropping of photos - second attempt

 

[MatthewC]

The very similar (but taller) lancet windows in our church tower staircase have been mortared in (with lime of course) but I don't really see why silicon wouldn't work in such a small amount.

 

[Gothichome]

Is your glass going to fit flush to the outside or stepped back a bit. My first thought was wooden pegs worked into the sides to hold the glass in place. If it needed to be sealed I wouldn’t go with silicone (the devils goo) but an acrylic calking.

 

[88v8]

Stone porch?
There needs to be a recess. It can't just sit flush to the outside.
stonemason could create a recess.
Then bed it on Hodgson's Heritage putty, and front it with the same.

Ivor