Double glazed unit - cat flap fitment

Morning,

Ive done a search but can not find the answer I need (if there is an answer) I'm after fitting a cat flap in a Double glazed unit. I understand I would need to replace the glass with a new one cut to size with the hole before its sealed..... etc.... but i'm told only a round hole can be cut in the toughened glass and my cat flap instructions require a square cut out...

The glass is approx 12" by 72" side light next to french doors...
Is it possible to have a shorter piece of glass put in the top so an in-fill plastic section can sit in the bottom with the cat lap cut out.... i.e is there a joiner/seal available to seal both the plastic and the new glass...?

I hope that makes sense??

Thanks in advance...

ones i have seen the bottom half of the door is 100% plastic.

If you take the flap to a local glazier he should be able to cut a square hole with radius corners. then send the glass away to be tempered, it is virtually impossible to cut a square without machining as the cut runs off we have done several with radius corners with no problems. but must admit round are much easier.