Temporary window treatment options? (shades, blinds, curtains)
If fashion and sophistication aren't high priorities - in other words, if you're primarily interested in function and controlling the cost - dark-colored flat bed sheets are something to consider.
The existing cellular shades probably have some kind of metal box at the top of the window to contain their operating mechanism. If that box happens to be steel then you can use magnets to hang the temporary coverings. Otherwise, you might consider hanging with tension rods or removable adhesive brackets (3M &quot;Command&quot; brand hooks).
You might also consider something solid like a sheet of thin hardboard, foam insulation board, foam core craft board, etc. These have the advantage that they could be painted to match the wall or in other attractive ways. The panels could be trimmed to fit the window opening so that they stay in place without hardware of any kind. They could be easily stowed behind a sofa, in a closet, or elsewhere to get them out of sight during the day.