So they are just glass panes that slide left and right and you push up to remove?
The unit you are buying is very heavy to it would have to be solidly fixed into timber at least, as I don't think even 6mm perspex would hand that kind of weight.
I don't know how handy you are, but I would personally make a lining for the aperture, then I would divide a section off on the left or right which would hold an 18mm board housing the air conditioner, and finally convert the other space to a casement window so you still retain the function of an open window. It might sound complicated but in truth it just a few bits of timber and a couple of hinges.