Please could someone suggest a way of doubling a 1/2f vco without using toko coils?

Regular air coils but will be an add ache to tune.

Have you made a doubler like that?

That was the way we always made them! Typically 4 turns at 6mm diameter with a 22p trimmer across it, stretched a bit. The base of the transistor needs to be biased just below conduction, so that the incoming RF nudges it into action. Some folks would couple inductively to the coil, with a couple of turns pushed into the 4 turn coil. The other ways were either to tap the coil for a take-off point, or connect to the collector of the transistor with a few pF into the next stage.

I also used this kind of coil for a tripler to Band II, using a FET instead of the usual BSX20 / 2N2369A.

I'll have a look in the junk pile and see if I can find an old exciter to photograph.

Incidently, if you can't get trimmers any more, you can use a fixed cap of 15pF, 18pF or 22pF, and stretch the coil to tune! It's fiddly, but it works.

Another option - one we're using - is to 3D-print coil formers. We then use aluminium grub screws to tune them.