OK, this is a little bit beyond me on how to build it, not because I'm not interested, but because I think I lack the necessary electronics background.
What is the principle of operation of a light meter? It uses a photodiode or something similar, doesn't it? Since a LED can also act as a photodiode (I tested this myself), can't you just use a LED, and shine whatever light on it and measure the mV's (often over 1V)? I did a few tests with a spare green LED, it required complete darkness to achieve 0.0mV (which implies it's quite accurate), and when I shine a lamp on it it can give 1.4 volts or more. It doesn't detect IR light, but an IR diode does (I tested this as well). I'm not sure what LED detects what light, but I would assume that it detects (peak sensitivity) whatever light it emits. The mV should be proportional to the mW of light received, so maybe this can be a good way to tell the approximated power of your laser.
I don't have a laser right now (old ones broke, waiting for new ones), but if you have:
2 same colour lasers (different power)
A power meter
A LED of the exact wavelength of the lasers
A voltmeter,
could you make a test on how accurate results this method gives? Like, shine one laser on the LED, get the mV/mW, then calculate how many mV the other laser should give (assuming they are proportional), then test the other laser.
Maybe this method doesn't have too many mW range, but it can be done
