Window sill with UPVC square bay window

[Commandpod]

I intend replacing an old external wooden window sill on a square bay. It has severely rotted on one side as you can see. I have a long plank of hardwood about three quarters of an inch thick. The depth of the old sill was less than that (about .5 inch). Will I need to reduce the depth so that the window sits on the new sill or can the gap under the window be filled with foam and the new top piece just go up to the edge of the UPCV window frame and sealed with silicone rather than underneath it?
What is the best DIY way of reducing the depth of the timber (hardwood) is necessary? Images below

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[Wayners]

Why not replace wood with something the correct thickness off the shelf then cover with uPVC.

Don't suppose you have access to a planer thicknesser to reduce your hardwood

 

[Commandpod]

OK, I'm going to get the wood cut and reduced in height using a thicknesser.
Am I okay to attach a wooden 'apron' around the bay (as in pic 2) directly next to the brick, or do I need to leave a gap (using small plugs to lift the wood away from wall? The old wood was directly against the brick.
What would be a good slope to have on the top? I thought 10%.