I have two sextants, both plastic and I consider the least precise model -- the Davis Mark 3 -- to be good enough and it weighs a whole lot less than a precision instrument in brass. Something to keep in mind is that on a small boat at sea you are not standing still; the boat is rolling and pitching always, it's what they do so the line you choose as your horizon is moving as they waves move you in the vertical plane.

Time is also a factor, you need precision in the time aspect of your shot because the look-up table expects you to know when the sighting occurred and it's better to take the shot at the time in the table rather than between times forcing you to interpolate. Really important when you shoot local apparent noon.

They sell cheap brass knock-offs online, but I wouldn't use one for actual navigation without extensive testing to see what it can do. The Davis Mark 25 is for if I ever buy that boat. Meantime, I can take sightings on sun, moon, stars and the occasional planet (Mars & Venus when visible) using the Mark 3 and really, the Mark 3 works.

Does the cardboard sextant come with sun filters?