Just buy a platypus. It will be easier to find the size you want, and to buy it. It will also be easier to fill and close.

A standard IV bag has a valve in the bottom of it that you can use to easily refill it.
(It's the yellow stoppered thing in the pic: http://www.isips.org/products/Graphics/Vial2Bag_needleless_system1.png)

You just need a syringe. That being said I would test the flow of the methanol out of the bag before taking it anywhere. It may have some difficulty making it out of the bag. Having never tried it, I am not really all that sure. That being said, IV bags are pretty expensive for what you want to do.

I am not really sure what would be a good alternative, as nothing pops to mind.

You can get Platypus softbottles down to .5L.

NB: Just because a bottle holds 1L, 2L etc doesn't mean that you need to fill it to capacity-- simply put what you need in, take the air out and fold the bottle to fit the space.

http://www.cascadedesigns.com/platypus/handheld-hydration/softbottle/product

If you had an impulse heat sealer, you could take almost any bag and make it smaller by resealing and cutting off the excess.

Also, another source of bags might be the grocery store. Look for stuff that is in resealable squeeze bags, like frosting.

How about an "Ayr Saline Nasal Mist" bottle. They are 50ml, somewhat flat and the top is removable so you can refill it.

I'm not sure if you can find these type of bottles easily.

We have a steady supply of these due to medical issues but currently we don't have any empties or I'd send you some.

You might could pack 2-3 inside your kettle.

Working with IV bags every third day, two issues pop to mind...

1) Seems like it would involve an awful lot of mess and extra effort. Replacing the fluids with denatured alky is possible, but would take a while. There is a "medication addition" port, but also a much larger port for the IV tubing to be inserted. You'd have to get an uused one, replace all the fluid with alky, and withdraw what you needed. If you got hold of a used one, you'd have to deal with the roller clamp on the tubing that adjusts the flow of fluid/medication. Thise things have a nasty way of "adjusting" themselves; on the ambulance I'll typically wind up getting the flow I need, then taping the roller clamp in place to prevent this.

2) Denatured alky might have some incompatability with the plastic used to make IV bags. Certain medications (such as nitroglycerin) need special IV sets because the plastic will absorb the medication. If you're seriously thinking about this route, consult a pharmacist to see if that will be an issue (and expect a weird look!). If it is going to be an issue, forget about it; meds that are absorbed by the bag are stored in glass bottles with special IV sets

Props for the creativity tho! It would be neat to whip one of those out at camp for the shock value, I'll admit