I have a bunch of metal water bottles.
Being a water bottle, it is inherently watertight. I'm thinking it'd make a pretty decent cache container.
However, it's ~220mm tall, and the opening is pretty small - 27mm.

So, I can roll up some A4 pieces of paper, and they'll fit just fine.

However, I am foreseeing people having problems getting such a log out of the bottle.
I can put it into the lid to start with (like one must with nano logs), but eventually someone's going to just shove it into the bottle proper, and then it's good luck to the next finder

Furthermore, I want to be able to have swaps and a pen/cil. (Finding a decent-sized container with no swaps is just disappointing, but to also omit a writing tool? That's crap and I expect better of myself.)
A log of this size would take up most of the room in the container.

Any ideas? Advice for/against this type of cache container? Anything else?
Discuss please!

24 pill containers, with the log in only one.

If you get a piece of tubing just smaller in diameter than the water bottle mouth and put your log sheet and pencil in the tube it is easy for the finder to remove and replace and protects the log from slipping sideways and being damaged during removal.
PS not my idea but a useful BobfromElsewhere (more commonly referred to as BFE) trick.

How about putting the log and writing implement in a large plastic bag that when replaced into the container has enough volume that it won't get shoved in too far and the bag gives you something to grab onto to yank it out again.

Don't do that. It will get shoved in too far. It will get torn when it gets pulled out. It will turn to rubbish within a few finds and remain as rubbish for ever. A container where the mouth is narrower than the container is never a great idea as the logs are inevitably not replaced correctly and end up more irritating than a nano.

Put it in the lid....

I agree!

Stick a bison tube to the lid.

I have this image in my head of those little flat cylindrical containers typically used to hold lip balm
And a cacher shaking the bottle at night while it's raining, making the most awful racket, muttering to themselves, "What is all this, why are there so many, crap another one rolled down the hill, oh my god where is the log?!"

I think this one would be best staying in my imagination...

This is what I'd do:

- Cut a piece of thin timber dowel to the same height as the bottle (without lid)
- Stick the logsheet to the dowel and roll it up like a scroll
- Secure with a little rubber band
- Pop that in the bottle for easy retrieval

You're not going to get many swaps in that container anyway. Even if it's big enough, the opening won't allow for anything significant to be popped in except the usual detritus of calling cards and 5c coins.

First thought: use them as water bottles and buy some open-mouthed thermos containers instead. I know, you want to use what you already have, but not every item makes a good cache container...

Pity that the neck is just a few mm too small for a 35mm film canister to fit.

Second thought: waterproof match containers should fit through the neck and provide a second waterproof barrier and will fit a micro-style log.