I would use a plinth block at the bottom corners of your window. Either make or buy the plinth blocks tall enough so your baseboard bumps into the side of the blocks. Along the bottom of the window, between the plinth block, you can rip the top off the baseboard to the exact height you need to act as a bottom trim on the window and fill the space to the floor. See this link for pictures of plinth blocks. It shows the block used in a door opening, but you should get the idea.....https://www.google.com/search?q=pic...uba0Cp98M:&usg=__JGmhy56rUwFbkbip8wO9H_WajEA=
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