When a concentration check is made you roll d20 and add your caster level and the ability score modifier used to determine bonus spells of the same type. (quoted)
If I was looking to create a magical item to add a bonus to this, what should be used? This is not a skill check – as there is no skill involved – although to my mind would be the most obvious category for it if such an item could be created.
So is there a way to create a magical item that would give a bonus to this check?
Yes, and there are several magic items that affect Concentration. It's probably best to back-figure from those as the default magic item creation rules don't always give good results for unusual items.
Check out (on the better PFSRD):
The gloves of elvenkind are the best compare (in fact, you probably should just use them and move along). Cloak/boots are also +5 skill items for 2500.
Now the gloves do Spellcraft and Concentration, so you have to calculate that using the "two effects on one item" cost - like Gloves of Swimming and Climbing are 6250 gp, which is 2500 (Swim) + 2500 (climb) + 1250 (50% of the second power combo cost).
Therefore the 7500 cost of these gloves is 3750 (Concentration) + 2500 (Spellcraft) + 1250 (50% of the second power combo cost). So Concentration bonuses could be said to be 50% more expensive than a skill bonus (bonus squared x 150 gp). Also, that bonus is only for casting defensively - I would assume that caveat would make a flat "bonus to Concentration checks in all circumstances" much more expensive. I have no basis for this than my opinion, but I'd go bonus squared x 500 gp - it's clearly intended to be less than a stat or save bonus, but more than a skill bonus.
In fact, there is also another more recent item that grants Concentration - Tunic of Careful Casting. It adds a +2 flat bonus to all concentration checks and costs 5000 gp. That is slightly more expensive than my guess at (bonus squared * 625 gp), but that should be pretty authoritative since it's a direct compare. (Thanks to @Hettikus for that find.)