Just about any sextant will do the job. Whether they are made out of plastic or metal, if they are properly calibrated, they will get the job done.
I am much more concerned about the eye shades on the sextant. I am an eye doctor, and over the years I have seen lots of people who have burned their retinas from sun gazing.
If you stare at the sun, it places a white burn in the very center of your retina - in your macula. The white burn can knock the central vision down to about 20/100 in the acute phase, and after the burn resolves, your vision may recover to 20/30, 20/40, or less, depending on how long you stared at the sun.
Most macular burns happen in people who are observing solar
eclipses through sunglasses or improvised filters like dark x-ray film that they hold in front of their eyes. (Unwise thing to do) X-ray film does not filter out the sun's harmful rays, and they come to me with solar
maculopathy. Drug users sometimes stare at the sun as well.
When I use a sextant for a sun sight, I start out with all the filters in place, and if I can't see the solar disc, then I remove the filters one at a time until I get a clear dull disc that does not leave an afterimage in my eye after I stop looking at the sun. The important thing is not to have an after image that lasts longer that about ten seconds when you are no longer looking at the sun. If you have a prolonged afterimage lasting more than thirty seconds after you gaze away from the solar disc, you probably burned your macula.
When I look at a sextant, the first thing I check out is the filters. If there are plenty of neutral density filters to protect my eyes, then I will look at the quality of construction, calibration, and price