Recommended settings and filaments

Filament type

PLA is your safest and easiest choice. It prints easily, onto virtually any surface, and doesn't require any enclosure. It boasts good impact resistance (you never know when your stroker decides to throw itself onto a neighbouring object), but when it breaks, it snaps.

Adding to its flaws, it does not like heat, and servomotors can produce quite a bit of heat when operating. PLA starts softening around 50°C (122°F), and deforms starting at 60°C (140°F). While your servos shouldn't be able to reach 60°C, especially if you add a fan, it is always better to know in advance.

PETG might be a better-suited plastic for this application than PLA is. While its impact resistance is lower, it doesn't break as easily and yields quite a bit before breaking. It is a more flexible type of plastic.
PETG also doesn't budge before hitting 80°C (176°F), which no servomotor should hit under our conditions.

Make sure to clear the path of the stroker during operation, and PETG should be perfectly fine for these needs.

ABS is overkill. Not only do you require a 3D printer with an actively-heated chamber to print it, and as little drafts as possible, but it is also harmful to your health to print, due to the amount of degasing happening during printing. It is not a bad choice of material if you are able to print it safely, but its mechanical and thermal properties are not needed here. If you truly want the properties of ABS but don't have a printer capable of printing it, ASA might be the solution.

Those are the three most common types of 3D printer filament on the market, and should all run you the same price per kilogram. More exotic filament, like PET, PC or PCTG are out of the scope of this page.

Flexible filaments, such as TPE and TPU are not needed for this, unless you want to print the 6mm gaskets for the OSR2 yourself.

 

Print settings

All parts can be printed coarse, at 0.2mm layer height. 

2 walls is a minimum, with 3 walls being recommended, as well as 30+% infill. For parts that need to be strong, such as the arms, it is a good idea to add a 4th wall and bump up the infill to 40% or more.

Line width should match your printer's nozzle's, typically, 0.4mm.

For temperature of the nozzle and bed, as well as the fan speed, follow the manufacturer's recommendations.