Edge ruler 6inch

For the most precise printing, to allow for slight shrinkage in the print on cooling, enlarge the x dimension in your slicer by 0.5% - i.e., set the scale at 100.5%. Or if, as I had to, you need to print at a 45 degree angle to have the parts fit one at a time on your print bed, scale both x and y by 0.3% (Scale 100.3%).

3D print the two parts, then glue them together using solvent cement to form an accurate right angle. A small ridge on one part, and a corresponding groove in the other, provide registration.