This is a bracket I made for a nominal 2x3 inch vinyl downspout.  The exact outside dimensions of the downspout was 59x79 mm.  The bracket is made from PETG to hold up better to weather and temperature variations.  A hole could be drill in it to attach to the building exterior if desired.  In my case it is simply attached to the downspout above an existing band to keep the downspout from sliding down.

The bracket is flexible enough to fit around the downspout and then tightly secured with the included bolt.