Connectors to sit on rounded surface
If you have access to a 3-D printer, you can create an adapter such that one surface matches the curve of your cylinder and the other surface is flat to match the connector.

Make the printed adapter such that it has mounting holes that match those in the connector.

You can easily make this waterproof by using sealing tape on both sides of the adapter.  I normally use Very High Bond (VHB) adhesive for this purpose but even silicone sealant will work.  This, of course, assumes that your connector is also waterproof.

My connector of choice for these (low cost) applications is Amp's CPC series.  There are silicone seals that fit inside the connector side with the male pins and the boot is available with a heat-shrink seal if the connector needs to be waterproof.

You can also use military MS series connectors if this needs to be a high-reliability application.  These are also available as waterproof.