If I wanted to carry meths in a holder on the bike, rather than packed away because I was irrationally concerned about getting meths over everything, does it need to be in a proper trangia bottle or would a clear plastic hdpe bottle still do?

I know it's unlikely but worried about it getting too hot if the sun was out?

Prefer a clear bottle so I can gauge usage better, though the valve looks good on the trangia bottle.

(off sick today so overthinking everything)

Should be fine in a plastic bottle. The effects of heat/sun (if it ever happens) won't be any worse than they would be in a sealed aluminium bottle ... in fact it might actually be better given that the plastic bottle won't have a sealed valve.

That was kind of my thinking, and given that meths is sold in fairly light plastic bottles (the 500ml bottle weighs 40g) it can't be dangerous. The annoying childproof top is easily changed for a flip top lid.