At this point I think a Schottky diode is just fine ...
I do not think the case of overly complicate the circuit with a MOSFET (also for the price).
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perhaps the mosfet does not have a high price then we could use it:
NMOS
http://www.ebay.it/itm/VISHAY-SILICO...item2a4f7f763a
PMOS
http://www.ebay.it/itm/transistor-SI...item5d4fafa18f

