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This is not a new idea - but here is my implementation. I bought a mini-moto fuel tank from ebay and attached a short length of 8mm pipe to the outlet then put a longer length of 10mm pipe over that and fastened it with a jubilee clip. I coiled a longer length of 8mm pipe and attached one end to the breather in the cap. To make the stand-off tank brackets I used 6mm stud with penny washers either side of the bulkhead and 1 3/4 ins of 15mm copper pipe plus washers and nuts.

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The 10mm fuel tap came from Ebay and the M24 x 3mm oil tank breather from Judd racing. Again, a short length of 8mm tube on the breather and a 10mm pipe over that plus jubilee clips. It means that the saildrive is full of oil but it doesnt need any expansion space because of the header tank. The pressure from the header tank prevents the ingress of seawater so when the seals fail, all you see is a fall in the oil level in the header tank. No panic. Replace them when you can but keep the tank topped up.