This is a 3D-printable dust cap for a Celestron 5SE (5" / 127 mm diameter) Schmidt-Cassegrain Telescope. (I bought my telescope second-hand, and the original owner had lost the dust cap for the main objective.) There are 6 little tapered "nubs" on the outside diameter which give a good secure press-fit into the end of the telescope.

You can easily adapt the design to suit telescopes with larger or smaller apertures by scaling the STL to suit - just check the clear inside diameter of the surround of your telescope's .objective, and scale the STL to ensure the "nubs" are just sightly larger diameter - the tapers should allow it to wedge in securely.

Alternatively, the original parametric CAD model is "Shared" publicly in my OnSpace account - search https://www.onshape.com/ for "Celestron 5SE Dust Cap".