Is chill in fermenter really that bad ?

[wide eyed and legless]

Why would a cube be any different from a sealed fermentor apart from maybe smaller headspace? especially if the fermentor has been fully sanitized?

The fermenter seals are not airtight, the closest retailer to you for jerry cans and cubes is The Plastic man, add about 3 litres to the volume 20 litre should hold 23 litres, 25 litre about 28 litres. If you go for the 25 litre cubes that should be sufficient, leaving plenty of head space, if you are pitching yeast the next day that will be fine.
https://www.theplasticman.com.au/collections/drums-jerry-cans