Ring of Continuation I hear.
AD&D 2E had versions of spells that boosted the spell durations by a similar amount.
Super Genius Games published a metamagic feat called Continual Spell that does what you describe. If there existed metamagic rods for metamagic feats that raised spell level by +4, you would have the desired item.
I frankly don't think this item exists, but if you ever do find it, please post -- I have been looking for ways to extend spells that change size and/or shape and so far have come up empty.
No really. Its called the Ring of Continuation.
This band of braided copper wires throbs ever so slightly in harmony with its wearer’s heartbeat.
Whenever the wearer of the ring casts a spell with a range of personal and a duration of 10 minutes per level or greater, that spell remains in effect for 24 hours or until the wearer casts another spell with a range of personal (whichever comes first). Spells that usually only affect one specific action (such as making a particular attack) are expended after that action. This does not reduce the duration of spells that normally last longer than 24 hours. Casting a new spell does not eliminate a previous spell if its normal duration is such that it would still be in effect. However, attempts to dispel or otherwise eliminate a spell kept active by the ring work normally and, if successful, remove the spell from the ring, causing the wearer to lose all benefits from that spell.
Its 56,000 gp.
Now I know how I missed it -- that item has some limitations that made it not work for the purpose I was looking for (casting Alter Self on a size Huge eidolon to make it Medium or Small until combat breaks out). But I think you answered the original question.
Rod of Extend Metamagic is always useful and might be helpful to you.