Arrange unpinned windows on left side only and leave right margin based on active pinned window.
Activate the pinned window and resize it to how wide you want the right margin to be.
Open Window: Arrange Windows… dialog, check Right Margin for Active Pinned Window checkbox and click OK.
? Right margin (empty region) will be left on right side. Unpinned windows will be arranged on left side.
Manually drag all pinned windows to the right margin so they are not on top of arranged windows.