vent dimensions on 1.3ft^3 box

If you are following sizes / dimensions from the manual, read it to find if the port displacement is taken into consideration or not.  The manual will tell you if the port is part of the overall box displacement specs.  The size of box volume you gave indicates that the port structure is likely not considered in that 1.3 cu ft displacement, but carefully check the manual to be sure.

In the case where you are designing by math and not following manufacturer's suggested size, the port displacement is entirely separate from that of the box's internal airspace.  Two separate volumes.  Port structure is not only the air inside the port but also the extra walls that are necessary to separate it from the box's internal airspace.

The port may be any shape at the opening: round, rectangular, triangular or other.  The importance is the consistency of  this opening shape throughout the length of the port structure.  And you must be able to accurately measure it and find its volume.  So for your question about the triangular opening:  can you use math that will tell you what the port volume is?  If so, triangular is fine for you to use.

And note:  you can always change the port opening dimensions, when following manual guidelines for tuning freq, as long as the square inches of that opening remain the same.  A circle can become a rectangle; a 12" x 2" rectangle can be changed to 8" x 3", etc.  It is when you change port opening size to a different square inch area that port length is thus affected (and tuning freq changes unless you change the port length along with the opening size).