Here are some containers that seem to work well for letterboxing
that I discovered in Korea awhile ago that might be available in the
US (possibly Asian markets in the US?):

Ko-Max 190ml 4.5"x3"x1.5" clear bottom with rust/red lid that
seals extremely well (not sure what brand this would be under in US
markets)

The Hangul (Korean text) on the bottom reads something like this:
buem tchil kyung-yung-chok-tchil-bup-ae whey-han-byo-si (anybody
who actually speaks Korean might be able to figure out what this
means--I can only read the characters--I haven't a clue what it says-
-I presume it's either details about the company or the container
itself)

Oh--it's good to 20 degrees below 0 Centigrade ;)

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Another awesome one is:

Tower--New Concept Seal Ware by Moon Il Co, Ltd
RL-1
Lot No MK311 260 mL
Good from -20* to -120* C

3.5"x5"x1.75" clear bottom, clear lid with grey/blue rubber edges
that seals extremely well also

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I absolutely love both of these containers!!!

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On a side note, I'm probably the only weirdo who loads her luggage
with plastic letterboxing containers instead of 'shopping wares'
from Korea on return trips to the US! The immigration/agriculture
folks at the airport must really think I'm odd!

dvn2r ckr