This wasn't really a rules question, Ring of Force Shield doesn't say you can do this, or even imply it, so you can't.
Reasons not to do this:
-Bracers of Armor grant a variable bonus that specifically calls out that it can be directed to armor properties.
-Ring of Force Shield only offers a flat +2 bonus. This bonus explicitly mimics a heavy shield.
-Allowing a similar item that grants an even larger bonus nearly obsoletes tower shields (even if the item can't be set for cover like a tower shield).
-Even if you restrict additional bonuses past +2 to item properties, you're starting to heavily encroach into the realm of actual shields. You will need to be very careful to price so that it is not an automatic purchase for every caster class. (the existing ring requires no proficiency since it has no penalties)
-Part of the issue here is the force effect that applies universally and is difficult to negate/ignore.
Pricing:
Going by Table: Estimating Magic Item Gold Piece Values, a "shield bonus" falls under AC: other, and uses a formula, bonus^2 *2500.
(I'm not actually sure how the price of RoFS is derived, it appears to be this, 2^2 *2500 = 10k, with an ad hoc reduction, -1500, for also having to wield the shield(?))
Pricing it as "armor" shouldn't be considered appropriate as the existing item clearly does not use that formula.
Even using that formula, you may ultimately find such an item to be underpriced.
Bracers of Armor work fine as a "slot swap" because you aren't bypassing the necessity to wield something (and they can't wind up significantly more powerful than standard armor). Hand->(anything else) can be a very powerful swap if you aren't careful (this categorically includes items like wands also).
Although, I'm in the camp that says the shield created by RoFS must also be wielded in a hand to grant the bonus.
For a magus, this means he shouldn't gain the benefit during any action that involves somatic casting or otherwise requires a free hand. (unless his other hand is available)
If you insist on effectively making this an "unslotted shield"(ie. unwielded), the price should almost definitely be doubled (even if it's on a ring, slightly more if it's fully slotless, IMO).
There's also the issue of maximum bonus.
I'm unconvinced it should be another +8, but I also suppose it's not entirely unreasonable. I might cap this item at +5 (requires CL of bonus x3 rather than x2), but I'd need to see how the item was being used by players.
I'm a little skeptical of the price using this formula.
For your example, a "RoFS of Light Fort", would cost 22,500g "base".
It's probably still appropriate to apply the 15% reduction, I'm not sure, that price is 19,125g. This puts it a little over double the cost of a +3 shield(the equivalent bonus), I think that's relatively fair given the (greatly) increased utility over a normal shield.
