Hi,
I have had quite a bit of practice----- I'm in the process of doing my 7th enclosure top.

This is done: ( the top to an 18"W x 18 x 24"H exo ) did it last night ):

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Find the end of the spline.
Carefully pull the screen/ spline using a needle nose pliers.
Most of the time it will pull the screen along with it.
Be sure to support the frame when you do this!

Be careful, the screen edge is really sharp ! The wire ends get me every time ! ouch

I get CHEAP picture frame type glass. ( you want as much light as possible to pass through w/out any uv protection)
I use 100% silicone to hold the glass in. It takes as least 5 days for the odor to dissipate from the silicone. You don't want to expose the frogs to it until it is completely dry.

A cheap (on sale ! ) ( plastic- frame and glass from a craft store works great).
The glass is thin ( little difficult to cut) but it's cheaper then having a glass guy cut it.

!!! the glass is slightly wider on one side that the other........ I think by about 1/8"

I keep PDFs in these. I do not drill additional ventilation holes !

I let the glass guy do that. I have 1/2" holes drilled in the lower back ( NOT the bottom ) for drainage from the substrate as it collects from the mister.

I have the glass ........to replace the screens with glass in my 2 tree frog enclosures. However, I will be leaving a 4 " section of screen on each. On the screen area is a fan that runs on a timer ( pulls the air out ) I have things running this way at the moment...but the screens are covered with thin/clear/ plastic poster/ frame stuff. I did have the glass guy cut this glass for me. I wanted it to be a little thicker so I did not have to worry about the ceramic heater near it during the winter.