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Not a biggie PSA but I just picked up a couple of these insulated stainless flasks from Lidl this morning:
https://www.lidl.co.uk/en/p/kitchen-kit/ernesto-insulated-flask/p36459
There were several different designs but these looked the best. They don't leak and just fit a standard bottle cage (well tiny bit small but still fit my metal cages which can be bent slightly). £3.99 each seemed a very good price as they are pretty sturdy.

Good work
Thanks! Do you think they would seal enough to hold fizzy drinks?
Look decent but I prefer my half litre one from Decathlon which has a strainer and large neck so you can drink your hot beverage straight from the flask.
Oops! It's only 0.35l
https://www.decathlon.co.uk/p/isothermal-stainless-steel-hiking-mug-0-35l/_/R-p-7233
Saw those, they looked a bit small (didnt check actual ML) but two at £8 seems good.
PlanetX do something similar in 750ml £9.99 each
Not sure if insulated or not but they’re stainless steel.
Will look out for them when next out. Quite like my Stanley trigger mug for carrying on Audaxes https://www.stanley1913.com/products/classic-trigger-action-travel-mug-16-oz
I’ve got the previous version.
https://flic.kr/p/2k1myA1
Fits nicely in the cage.
Packaging and description say it should be ok for carbonated drinks.
@shedbrewed Nice Maverick! I nearly bought one of those last year secondhand to use as a commuter/gravel bike but just missed out which was a pity as it looked like a great long distance ride.
Thanks @winston it’s been around in a few guises since I got it new albeit reduced considerably. There’s only the frame and fork of the original Maverick left now as I upgraded it as it went along. The shifters and brakes are now on the wife’s GT GRADE and the hand built wheels are on my trek Crockett that is now used for audaxes. If I get hold of some more cable discs I might rebuild it as I’ve got all the other parts. I’ve been looking for a replacement pair of forks for ages as they are very heavy; 1.5kg! The frame shares A LOT of design and parts with other more expensive frames out there, which makes me smile.