Hello Pearl,
I have no idea about what your physical condition is - as in, why you need to be in a couch and whether you have restricted mobility of any kind..
I would just say that IMO, the idea of playing a theremin with it on a tray on your lap, well - its not going to work - there is simply too much movement in the body to make such an idea workable.
IMO, you need the theremin supported - some floor-standing table or mobile tray type support - the kind of thing used in hospitals that provide a 'table' patients can have their food on when confined to bed - that sort of thing..
Edit -> (I have just seen that Christopher covered the above and linked to a supported tray that looks ideal)
I actually wonder if perhaps you would be better off with a ribbon controller and a small analogue synthesiser.
http://www.soundonsound.com/sos/may00/articles/kurzweil.htm
https://www.youtube.com/watch?v=0XHSxeF0CxI
But a lot comes down to your physical state - you certainly can play a theremin while seated, but I dont believe you could play one with the theremin in any situation where your body is supporting any part of it or what its on.
The other thing you could perhaps use is a tablet PC with touch screen - There are some apps that allow one to play the screen - I have a tiny android tablet and although interesting, its not really playable - but I believe that a large Ipad or similar may give you a playable surface -
http://www.thereminworld.com/Article/13295/roundup--theremin-apps-for-the-iphone
Finally - Dont under estimate the physical effort involved in playing a theremin. I suffered heart failure some years ago, and the effort of extending maintaining the arm / hand is surprisingly tiring - 5 minutes is all I can manage now before I start getting heart irregularities.
Not trying to put you off, but just giving this warning in case you have potential problems. IMO the theremin is not the best instrument for people with restricted mobility - Oh, as with everything, a determined individual can overcome their physical limitations - but I am inclined to think it more sensible to pick an instrument you have a chance to excell in and master, rather than one which will present a frustrating barrier at some future time - You may think that you just want it for fun.. And this may be true .. But it may turn out that this fun becomes "bigger" and you want to progress.. Picking the right instrument at the start might enable you to do this.
Fred.