You have a bunch of options. The first is to buy 2 front brakes, since many sellers offer calipers individually. There's no assurance that a front brake will fit your unrecessed rear but the odds favor you.

Or if you can find calipers you like with long bolts, buy 2 rears and use on in front.

Or buy a matched set front and rear with long bolts, it might have enough thread in front to saw off some and use the shortest recessed nut you can find. Or if you must use the longer bolt with a nut, buy a recessed nut, drill it out and use it as a centering and support washer for the regular nut in back.