If it's OK to have clasps on the outside, 3 or more of these
or similar would be my choice, together with a simple gasket.
O rings are comparatively expensive, especially in larger diameters and they need a carefully machined groove. A simple gasket can be made from any kind of "rubber" that'll handle the environment it's destined for - perhaps just cut one from an inner tube.
You can also make a gasket mounted on one of the mating surfaces, by smearing a generous amount of semi-fluid RTV silicone on it and resting it a few mm above something non-sticky, like Teflon baking paper or the carrier paper for self adhesive labels. You just need some way of holding the body that takes the gasket above eg. a table, at the distance that gives the correct thickness.