Just make a mockup. Stand the sides of the cab up at the distance of the cab's width.
Place yourself in front of the 'cab', hold the monitor at the height/angle that is most comfortable for you, mark the position on the cab's sides.
Placing the monitor onto something, a pile of books, whatever, does help.
Note that standing in front of the cab and sitting in front of it makes a difference on the viewing angle, i.e. you'll get a perfect viewing angle standing at the cost of a not so good one sitting, and vice versa.
Just experiment until you find your best placement, there's no general rule for placing the monitor.
It will of course look best if it sits squarely between the sides, but if that means that you don't get the best viewing position, I wouldn't do it. In the long run, it will be no fun to have a cab that looks terrific but gives you neck cramps after playing five minutes.
And finally, before doing anything permanent, just post what you are going to do and see what the folks here have to say about it, they will point out any gross mistakes
