Outdoor finish

I am in the process of making a plinth to carry a memorial bronze plaque for my local RSA. The plinth will be mounted in front of a Gallipoli pine sapling which is a descendant of the famous Lone Pine.

I plan to make the plinth from two pieces of 100 sq. treated timber (ground quality) which will be planed smooth and jointed to make a slab roughly 200 x 100. The top will be chamfered at 30 degs to take the plaque. The slab will then be concreted into the ground with about 150 - 200 exposed above ground.

The timber I want to use has some small checks in it which I want to fill to prevent moisture ingress then I planned to finish using Aquamax Black linseed oil/water stain.

My question is, what can I use to fill the checks which hopefully won't shrink too much and will accept the stain and/or should I be considering a different type of weatherproofing finish?
Pete