Not sure I fully understand what you want, but I think you can use the same rect that you use for FillSolidRect and create a rectangular region (CreateRectRgn). Then, select that into your DC as a ClipRegion (SelectClipRgn). Now when you draw text (or anything else), your drawing will only occur within that region.

Be sure to select the NULL region to re-enable drawing in the entire DC.

Hope that helps.