ok, so you need a 3/8th to 3/8th riser.
I recommend THIS one:
BT Fixed Sight Rail
this will get you enough rise to use the sight with a mask on if you intend to use a stock.
if you don't intend to use a stock - forget the sight riser rail. install your sight directly to the 3/8ths rail on the A-5 and remove the stock iron sights from the gun..