coil basics

"... but i hadn't yet made the connection that the ferrite is what caused the drift."  -xoadc

Ferrite concentrates the magnetic field and there are formulations that aren't too temperature dependent, the trick is finding suitable inductors that use the milder ferrites, or finding forms made of it that you can wind yourself.  Like with capacitors, the formulations that concentrate the field the best and therefore give you the fewest turns and smallest inductor seem to be the most temperature dependent.

"am I right in assuming I can wrap my wire around any material as long as it is round and not a conductor?"

The relative permittivity of the form material mostly influences the self capacitance.  Any temperature dependence of the geometry of the form material is probably more significant.  PVC, wood, heavy cardboard, phenolic tube, etc. should work.  I like PVC because it's easy to purchase, is easily worked (cutting to length, drilling holes) and is really rigid, though it can be a bit heavier than the alternatives.

I've finally gotten all my materials together!  Ready to go!

one last question.... how to tap the coil.
I've been looking online, and most of the tutorials and videos that I've found are for tapping guitar pickups.

This coil, has 100 turns, and it's tapped at #20.
(I'm coiling around old plastic film cannisters).... how do I make the tap? 

My guess is that I just sand off a spot, and solder on another piece of wire coming out there?
Is it that easy?

don't sand a spot. rather like here: fig 2.1.2 . and a rough how to, like here: fig.3.1 or here: tap-it .

(could not get the picture work, so there are some links to them.)

Woah!  Beauty!  Thanks, those diagrams are perfect!

So according to fig. 2.1.2, I can use 2 separate pieces of wire:
wire #1 = wind 20 turns, and pull out a long piece at the end - wind a piece of wire #2 around that dangling piece of wire #1, then resume turning

wire #2 = continues 80 turns after it's connection point with wire #1

or.... according to the "tap-it" link... I wind 20 turns, pull out a loop, twist it, and wind another 80 turns.

Both methods seem pretty straight forward.

Thanks!

My advice is buy a cheap ~$32 LC meter from China and eliminate most/all of the guesswork.  Search for "LC200A" on eBay.  I've got one and it works like a champ for the small C and L you'll encounter in Theremin design.

Finally made those coils! 
(I got sidetracked the last couple of months as a few contracts came up... but I'm back on track now!)

here are the coils. right now they're just held taught with masking tape... but I should probably seal them with varnish or something right? 

now to assemble the circuit

(you can see my etherwave case still unbuilt in the background... I have been testing various materials for drawing and staining on scrap pieces of ash before committing to the real thing... but I carved it yesterday and I'm ready to stain it tomorrow!)

and here are two of them!