My umbrella stand has a 55mm internal diameter and a 57.5mm external diameter so I have designed around those dimensions. This is designed to sit inside of the internal diameter. 

My umbrella stand also has a screw to lock in the umbrella. I have left an arrow on the top to guide the user into the screw hole. I have left the hole out of the design to make it more flexible for other users. Given that drilling a single hole is pretty simple. 

Printer Settings:

• Infill: 15%
• Print Settings: 0.15mm Quality
• Filament: Overture PETG

I did it on quality because I wanted the arrow to look nice. If you don't really care too much about that then I'd go for 0.3mm to get it done faster.