Reductor that done from identical gears 1:4 each (uniform) and can be setup with as many gears in sequence as you need (scalable) to get ratio and torque that you need.
On images reductor done from 4 gears and has ration 1:256, but you can easy install more/less gears to each of two reductor axes to have ratio that you need.
There also included fram for gears system with holder for motor.
Print as many gears 1:4 as you need (file "Gear 10-to-40.STL"). Then print 1 (or two if you want to make it symmetrical) detail "Motor Holder (Opposite).STL" - this will makes frame (you can also adjust and print detail "Motor Holder (Top).STL" as opposite side of frame). Use bolts M4 to connect frames with gears inside. First gear put just on frame, before putting second gear print detail "Motor Holder (Rail) 7 mm.STL" and install it on bolt M4 first (it will makes correct shift in height before second gear). Then put all other gears you want and finally put one more detail "Motor Holder (Rail) 7 mm.STL".
Frame has 3 holes to connect frame parts outside gears axes - print 3 details "Motor Holder (Rail) 37 mm.STL" (for 4 gears 37 mm will be just fine, if you use other number of gears you can adjust height in file "Motor Holder (Rail).SLDPRT" taking in mind that every gear is 12 mm height) and install them into remaining 3 holes of frame.
I also attached sample holder for motor that I have (28 mm diameter) and adapter that makes possible install first gear with 12 tooth (small one on images). Probably you'll need to adjust those details according to motor you have (you can do it in files "Motor Holder (Top).SLDPRT", "Motor Gear Connector (Sliced) - 2.SLDPRT" and "Metric - Spur gear 2M 12T 20PA 12FW ---S12N75H50L5N.sldprt"). I also attached sample small gear with 10 tooths (file "Metric - Spur gear 2M 10T 20PA 7FW ---S10N75H50L5N.stl").
Note: if you want to print directly from .STL file you'll need to flip Y-Z planes for some details to place them correct on printing table.
Good luck ;)