Make dimensions common for all variants in Craft Commerce
This is actually one of Commerce's best features IMO - that you CAN have separate dimensions/weights for variants - for us this is absolutely essential.
Of course, there are definitely times when it is not needed, too.
In these cases, we have a 'Standard Box' section that we store standardised entries in, with the appropriate data.
We then just relate these to the variant using a relations field.
Works really well...however, the behaviour is of course not the same as using the in-built fields - e.g. your shipping calculator needs to be aware of this etc.