I will be removing some old wood sliding windows on my house and replacing with new vinyl sliding windows.The house is sided with hardboard siding. After removing the old windows I will have appx. a 1-1/2-2 gap around the new window on each side. I was thinking of filling each side up with plywood to bring the depth even with the siding. Since the siding will no longer butt against the new windows on all four sides, I was considering putting up shutters on the sides to cover up the plywood. And build a window flower box to cover the plywood on the bottom. Does this sound like a good idea? Any thoughts on what I could use to cover the gap on top? I also do not know what I will find under the siding. If there is nothing under the siding should I slip some roofing paper behind the siding, and if so, how far behind the siding? Or can I just use flashing tape directly on the wood?