" You have purchased your instrument already assembled, so what you need to do now is to make it your own by customizing the box." - Coalport
IMO, Customising is best only done after you have played a bit with your instrument and got some feel for it - And I think this is particularly true for the theremin. Assuming your theremin is working reasonably well (is correctly tuned etc) I think it is useful to first establish what is "normal" operation before you mess with anything - because otherwise you wont know if your customisation has had any detrimental (or positive) effect on playability or sound.
With most electronic musical instruments (synths etc) it is quite simple to take the guts and transfer them to a upgraded enclosure - you can take a Yamaha mini keyboard synth for example, and fit a full size keyboard and change the switches and actuators and sockets, and create a full size monster, without much likelyhood of getting weird problems..
But with a theremin, just moving anything or changing the track of one wire, or adding a couple of screws, or painting the box with the 'wrong' paint, can completely change its behaviour and at least require re-tuning of the inductors.
In the end, I think it all comes down to how you are "tuned" -
If you are technical and understand the processes (particularly capacitance) involved, you should not have too much trouble..
If you are "instinctively technical" or have loads of patience and are systematic and rational, and can follow instructions given on-line regarding tuning the EW - and can gain the "understanding" to allow you to re-tune the instrument, you should be ok.
But if you dont have the above qualities and plough into building a box and customising your instrument, then you do risk ending up with inferior operation.
Looking at Coalports wonderful creations and customisations, it is clear to me that he is HIGHLY skilled in these arts - Skilled in the crafting and integration of mechanical and electronic parts to create something truly beautiful and functional...
But all of us are different -
The above is not meant to be discouraging - I think Coalport's idea is great and certainly believe that familiarising yourself with the stuff that makes your instrument and learning / understanding / customising it is by far the best way to "bond" with it and get the best interaction with it.
But I also advise caution - I would avise small steps, starting by getting familiar with it in an unmodified state.
Fred.