Marc,
the idea of a 'floating lid', like in oil tanks, is rather good!
Not sure the effect of unprotected styrene in contact with printers ink would be, long term (ie, greater then 4 - 9 months, for example).
So for materials, I would suggest something that fits the design of your idea, strong & flexible, yet relatively immune to the effects of the 'grease' base.
Materials? Maybe a backing of grease proof paper (sometimes used in baking liners) perhaps? [that is, this is what our suppliers used to use when we opened a new can of ink- the lid, a trimmed sheet of grease proof paper, and sometimes then there would be a skin of dried ink]
With out knowing the dimensions of the neck of the jar, couldn't say much else.
I think your idea has merit, Marc. feel free to post a visitors message, Ill share what I can!
Soldierman, apologies if it seems like we hijacked your thread. Going by the views count, it seems to have generated lots of interest. Hope the additional comments have been useful
cheers