Okay, I've come up with what seems like a fairly reasonable solution:
From reading around, it seems that 0.6l Sigg fuel bottles are an endangered species. It also seems that un-coated aluminium water bottles, while fine for paraffin or similar, don't get on with methanol long-term. The Trangia bottle top is highly recommended, and having had a play with it, this is for good reason - it really does make pouring easy.
On this basis, when I came across a 0.5l Trangia fuel bottle on sale for about the same price as the lid by itself, I went ahead and bought it. For reference, this bottle is made of polythene and is rated for "fuel alcohols, kerosene and gasoline", but should not be pressurised.
Now, this bottle is significantly narrower than a standard water bottle, so how best to carry it on the bike (I dismissed "in the panniers" as tempting fate, though it does seem reasonably leak-proof)? The
Topeak Modula EX bottle cage seems to be highly regarded as a solution for non-standard bottle-shaped loads. At £3.99 including postage, I bought one too.
The adjustable mechanism works as intended, and can be set to fit the Trangia bottle perfectly. It should also do a reasonable job on most disposable soft drink bottles you're likely to encounter. The only problem is that it has the usual lip, which the Trangia bottle lacks a notch for. This isn't actually a huge problem, as it means the bottle just sits at a slight angle, the adjustable mechanism holding the bottom end securely. However being a perfectionist (or at least a botcher, I'm not sure which), I dug the Dremel out and had a go at the lip, cutting it down by a centimetre or so into a slight 'V' shape. The result is a bottle cage that fits the Trangia bottle perfectly, and can be adjusted to take water bottles as needed, for a lot less than the cost of a
Bikebuddy:
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