One problem with making a new sensor for a cheap lux meter, to convert it into a PAR meter, is the difficulty of reproducing the parts so they are so identical as to get the same result every time. There are so many variables that affect the sensitivity of the sensor to light that it would take machine shop precision to be able to make each sensor work exactly alike. Most of us, me included, don't have a machine shop. I have a hacksaw, a hand drill and sandpaper instead.
In this thread, DIY PAR Meter
Jalopy proposed an alternative to having a machine shop - use an online 3D printing service to make the mechanical parts. That has inspired me to revisit my DIY PAR meter design.
I modified Jalopy's sensor housing design until I got it down to the smallest and least complicated shape that would fix the critical dimensions so each part would essentially be identical. DIY PAR Meter
I wanted it to be the smallest it could be, because 3D printing is priced by the volume of material in the part. Even with my "smallest design possible" version the housings cost $18, about $9 for the part and $9 for shipping. That is a significant cost for such a tiny part.
A bigger problem is the limited set of materials that 3D printing can be done with. This housing has to act as the cosine diffuser as well as containing the other parts. A good material for a diffuser will be white, it will not absorb or reflect differently for different parts of the light spectrum. It will also be the same for each part that is made, and it will have a transmissivity that allows the diffuser section to be about .05 inches thick - thinner would be too prone to mechanical failure and to damage during the cleaning process after the part is printed. Thicker would prevent limiting the light striking the photodiode to just the diffuser area since the whole housing would be made of the same material. It also has to be waterproof, not so porous as to allow water to quickly leak through it. I can find no such material among those that Shapeways, the 3D printing-for-DIYers company, has available.
Jalopy mentioned to me, in a PM, that he was looking at the possibility of molding the housings out of plastic. That started me looking at the possibility of casting the parts.
Amateurs can easily cast parts from polyester resin or epoxy resin, but not acrylic resin, which is ordinarily used for optical quality parts. Polyester casting requires mixing catalyst into the resin a few drops per ounce of resin, with the number of drops being determined by the thickness of the casting and other characteristics of the part to be cast. This makes making identical casting very difficult, since each part uses less than 1 ounce of resin.
Epoxy casting requires equal quantities of two resin components, a much easier to control mixture, and not a function of the thickness of the parts. Clear epoxy casting is well within the capability of an amateur, like me. So, I started teaching myself to cast epoxy parts!!