Introduction: Illuminance Increasing Window Reflector
Shortest possible summary: I had a dark basement. This helps a lot; perhaps a 50% improvement on cloudy days.
I had a window in the basement with a shelf in front of it, and most of the light which came through the plane of the window did not make it into the rest of the room, as it hit the (dark) windowsill and was absorbed or, after bouncing off the windowsill, went in some lousy direction like back out the window. Ideally, we'd build a nonfocusing optic (compact elliptical reflector) designed to make sure that all light would miss the shelf and the ceiling above it, but we don't have the vertical space to do so, either in the window frame, or in the region above the shelf, so we do the best we can.
Because the reflectors are so short, there's little point in making them curved, while that would've substantially increased the complexity of construction and design, required raytracing to optimize their shape. So the reflectors are flat.
The reflectors are angled as steeply as they could be... which isn't very steep. The bottom reflector is angled perhaps 5 degrees, and the top and side reflectors perhaps 25-30 degrees.
If you have a very large window opening, in relation to it's depth, you might find that you could angle the reflectors much more than this.... but I wouldn't recommend it. I'd keep the angle of the reflectors to about 40 degrees, as this angle will deflect even the steepest rays (say, 80 degrees to vertical) to a horizontal direction.
The reflectors are fabricated by cutting the correct shapes out of cardboard (trapezoids with a height equal to the length required to go from the window to the end of the windowsill, with a width which goes from the window's dimension to the window-hole's dimension), applying polyurethane furniture finish, laying aluminum foil on top, brushing with a broom to smooth the aluminum foil down, and then letting the polyurethane dry. These are then trimmed and taped together, being careful not to peel the aluminum foil off, as it isn't adhered that well - it seems polyurethane doesn't work very well as an adhesive for aluminum (or I didn't use enough).
See also: https://www.instructables.com/id/Cardboard-Foil-Glue-The-Solar-Funnel-Cooker/step3/Apply-foil/