Similar to above, got a red sea reefer 250 so a 10% weekly water change into a 25 litre container is perfect for me, so i filter my RO straight into one of these.
For mixing got myself one of those large flexable gardening buckets from aldi (think 30 or larger litres).
When I get more or update the containers I'll be going for anti glug pourers, this would help when pouring RO into mixing bucket.
If your mixing bucket is near your tank you may want to consider a water pump to exchange water from that into your tank / sump.