My solar lights are too narrow (30.4mm) for the post hole (37.7mm), so I designed a cap that supports them and accents with color. 

Added a stem to hold the light, replacing the original stake.  Fits nicely around the light base inside the cap.  Dimensions: 17.9mm o.d., 15.3mm i.d., 55mm length

43mm (1.7") inner diameter, post cap for your marine dock with 18.3mm hole for solar light. Sturdy and prints well in PLA.