Picture a low, flat box topped with clear plastic, recessed into the ground (for insulation). There are two flat mirrored sheets which attach to the east and west edges of the box, hinged so they can pivot independently. Each is the same size and shape as the top of the box. Like so:
/
/
west / east
/ -----------------
----------------+===================+-----------------
regolith | plants | regolith
|___________________|
The east mirror is drawn lying flat on the ground. During the morning, the
west mirror reflects sunlight onto the plants in the box, increasing the amount
of light they get (and also warming them). Throughout the morning, the west
mirror pivots to keep the reflected sunlight centered on the plants. After
noon, the west mirror isn't very effective at reflecting sunlight into the box,
so you lower it to the ground and raise the east mirror and use that until
sunset.
After a bit of geometry and a bit of math, I've found that you can effectively double the amount of sunlight reaching the plant box using this mirror arrangement, assuming the sun passes directly overhead. This would let the plants get as much sunlight on Mars as they do on Earth, and would raise the (assumed black-body) temperature by a factor of 20% absolute, or about 50 C! This would make daily mean temperatures inside the box around 280 K, based on Mars Pathfinder temp data. Daytime highs could reach 315 K!
The best part is, during the nighttime you fold both mirrors over the glass box. The mirrors now hold the long-wave thermal emissions in, keeping the box warm during the night. A little fiberglass insulation on the backs of the mirrors should keep the temperature above freezing at night, though I haven't worked out the physics. If this reduces nighttime cooling by a factor of two, nighttime temperatures in the box would get down to 265 K. The mass of plants and water vapor in the box and the surrounding regolith should moderate temperatures in the box nicely; I'd guess temps would stay between 280 and 300 K, perfect for plant growth.