Okay! How about this;
I ordered some surface-mount LEDs from Ebay/China, the wavelengths I am missing. Unfortunately, they always sell at least 10 to 20 pieces, whereas I only need one of each . And I have hundreds or thousands of some wavelengths already.
I happen to make PCB's all the time and it would be wasteful to make just one of those tiny little boards. So, I will make several of them at the same time. And once I need to calibrate mine, I can just apply that calibration to all of them. I am thinking about fixed resistors.
So, those will be tiny little units with a series of LED's: for example, IIRC, 660, 625, 600, 580, 550, 520, 470, 400 nm. As they are LED's, they are not sharp but show some response to +/- 25 nm so there is a bit uncertainty there, and the calibration won't be perfect either, but at least it will be cheap, small (something like a credit card) and simple and quickly done. It will need some kind of diffusor, but I have a solution for that ready too. It will work from a 9V battery and cost something like 5-10 USD.
I can include a simple timer to the design with practically no extra size or cost.
Anyone interested? What about the dimensions? I was thinking the LED "bar" could be something like 5 cm (2") long.
I will build this one for myself anyway and this is easy to "mass produce" in a range of 1-10 units.