I have a better idea. Buy the party arcanist a 3rd level pearl of power or memento magica. Have him cast Greater Mighty Wallop on you every day. If he's nice, he may be willing to expend one measly 3rd level slot to buff you without buying the item at all.
If you did make a ring, by the very flawed DMG rules, it'd be something like 1800 or 2000 gp, standard action to activate, and would last for 1 minute (CL 1).
To make a ring with a continuous MW, the DMG would say :
Item Cost = + Material(50) + [XP(50)5] = 0.
Spell Effect Cost = Spell Level (1) x CL (1) x 2,000GP x 2 (modifier for being a spell with minutes/level duration) = 4,000.
For a 1 Use/day effect, it would cost 1,800. Spell Level (1) x CL (1) x 1,800.
Because of the usefulness of the spell, I'd argue that a once per day item would NOT increase the item cost, but a continuous item would cost 1.5x extra (4,000 x 1.5 = 6,000).
As far as Greater Mighty Wallop would go, the cost is dramatically increased, especially at maximum potential.
Minimum, a continuous ring of GMW is; Spell Level (3) x CL (5) x 2,000GP = 30,000 GP.
If it is at maximum effectiveness (assuming Medium creature is the base); Spell Level (3) x CL (20) x 2,000GP = 120,000GP.
Is it worth 120k?
YES.
Is it the best way to spend 120k?
Maybe not.