Nerf Titan Barrel Adapter for 1/2" conduit

This is a very simple adaptation of RhinoAus' [Titan barrel adapter](https://www.thingiverse.com/thing:2291685). I've changed the size of the hole such that it should work with 1/2" conduit, which has an actual OD of 0.840 inches.

Potentially confusingly, this conduit is sometimes referred to as 16mm - but 16mm is the nominal ID and therefore RhinoAus' 16mm adapter is not suitable.