Brilliant. I like your thinking Lee. One or two problems I can see that you would have to have overcome - the increased size of the shadow and keeping it clear of shavings, but an eggsellent (sorry) solution to the problem, and no more or less cheating than a plywood template, which is accepted standard practice.
I kind of have solutions to both those issues...as I anticipated them before setting up so came up with the following...
You are correct about the size of shadow changing...however, I knew I had to make quite a few of these eggs as I have lots of similar size offcuts. So I worked out how thick I need to rough each blank to round and made sure that each blank was roughed to the same dimension. I then made sure that the light was fixed too, and couldn't be moved. I then measured the shadow width. Once I knew the width, I got a vector of an egg and resized it to match the shadow width. When everything is in place, all i now need to do is ensure that each blank is turned to the same size and everything is good to go. I quite like this idea of the shadow tracing (not sure what the actual technique is called) so I think i may make an adjustable platform rather than the fixed one i made.
Regarding the shavings, a small fan fixed in the right place is good for keeping the image clear. Normally Id just blow away the bits, but I'm starting to wear masks now so I can't...plus Ive been wearing glasses but think Ill switch to a visor...so definitely no blowing away the swarf then! :)
My advice to anyone trying it would be just to be aware and careful when following the shadow...a couple of times I dug the tool into the wood as you kind of have to have an eye on both the piece and the shadow...so you just have to be a bit careful.