Copied from the official website:

1515 is a 1.50” x 1.50” fractional 15 series square T-slot profile with four open T-slots, one on each 1.50” face. The profile has align-a-grooves to assist in alignment when connecting profiles. The 1515 profile is compatible with all 15 series fasteners. This profile is the most heavy-duty in the 1515 profile family and lends itself to clean room applications, guards, enclosures, or desks.

This is a 1 inch length bar of the stuff. What I do is scale this model in Z to the length that I need before printing, so that I don't need to regenerate the model every time. It's not quite as strong as the aluminum version :)

The STL is in units of mm, so should come out dimensionally correct with all the common slicers.

Drawing dimensions: https://8020.net/1515.html

A 6" bar with 20% infill took 8.5 hrs to print.