Just use 1/4 ply instead of 1/8 for the top of the lid.  My window is 15cm x 20.5cm centred on the tracker bar.  The important thing is to use a "smoked" plexigass, otherwise if it is too transparent the window appears as a big hole in the organ and the aesthetic line of the organ is "broken".  It is interesting to see inside, but not too much; just the punched holes!

"Smoked" plexiglass can be chosen the same colour as wood and looks very nice.

Why Enlarge the Window?
Just because of the fascination of the tracker Bar.  Let me tell you about this personal feeling:

I am not sure of the conditions when I discovered mechanical music.  Was it an organ or a player piano, or maybe a fair organ?  I do not remember.  I was young.  What I remember was the fascination I had in front of the books running, so it was probably a fair organ.  But, even now, I can still say I did not lose the fascination! And I bet I am not alone when I see youngsters discovering instruments, and the correspondence between the eyes and the ears. What might be their thoughts at that moment?  "So, holes are music?"  "I got it! These holes are the low notes, those are the high ones!"  "That line in the middle: they call it a tracker bar." "So distance is time?"  "Let's have a look at what's coming next: is that the future?"  "How will that funny pattern of holes sound when it is played?"  "What will all those holes together do when they get to the tracker bar?"  "If this is the future, I can see the future, present and past all at one time, at one point!"

But it is amazing to notice that watchers only glance downstream; they always look upstream as if not interested in the past. Everyone is awaiting something from the future, don't you think?  At those moments it seems that only the elders pay attention to the music itself!  The punched paper roll flows over the tracker bar just like a river does, and the magic is nearly the same. People would rather look upstream and follow that fallen leaf or this boat along the stream than to look straight ahead at a fixed point, unless they are looking inside themselves.

Time flies by! Would organs also be musical sand-timers? "Tempus fugit", as is often seen on clock dials, but the clock pendulum swings one cycle and gives the impression of a permanent new start.  With punched music, what has gone has gone, but if the refrain comes, of course! Maybe this is why the magic is not the same with barrel organs and musical box cylinders (which are also much slower and with less contrast of pins).

This fascination of punched music is an important part of the feast.  Doesn't it look like a rain of confetti, providing that you can watch it in a wide field, at 'wide angle'?  If you can look at the line of the tracker bar only through a narrow window, you can't follow the holes and the fascination disappears!

With MIDI systems, part of the feast is forgotten! Never mind.  I am sure someone is going to outfit a computer screen to represent paper running by!

At this very moment, while I am writing, snow begins to fall, and it gives me the opportunity to go further... Looking out at the slow hazardous fall of a snowflake is an enjoyment, but looking straight away through the flow might incur some sadness, couldn't it?  So, is that fascination some of the curiosity and freshness of a child discovering life, or a predator's behaviour attracted by anything moving and ready to jump? I do not know. Ask Freud and Darwin!  I beg your pardon; I did not intend to go so far with my tracker bar!

Jean Nimal