Mirrors are lousy reflectors. A good white paint would reflect more light. If you can shape a piece of thin aluminum sheet in a \__/ form, and attach that with one bulb per reflector, you have about the best reflector for a T8 bulb that is possible. There is room under the bulb with most strip lights to do this. With the reflector installed, if you look up at the bulb, you should see 3 bulbs instead of one - that is the extra light you can get from the sides of the bulb. The only problem is if the bulbs are too close together to get a reflector for each bulb in between them. Also, you don't need to do a super polishing job on the reflector for this to be very effective.If you want easy and cheap, there's always aluminum foil. Slightly more expensive is a handful of small mirrors from walmart or target. Tape or glue the foil or mirrors to the underside of the fixture.
