Re leaving at home, wherever possible we like our dogs to come too. They are part of the family after all. Our old girl Betty even used to go roller blading with my wife, frequently entailing running up to 10km non-stop. And she kept this up even when she was 16 yrs old! Beware that cockers and roller-blading are not a good match and it took a good number of tumbles and crashes before Betty got the hang of it.
Basket. As mentioned in OP, I'll eventually get round to posting a pic. What we use sounds horrible but its completely effective: On the back of my mountain bike I have a standard bolt-on pannier-rack. On the top of the rack is mounted a square wire shopping basket ( I can measure it if you need dimensions). Of course paws and tails( if he had one) would always falling through the basket and getting tangled in moving parts of the bike, so I've lined it internally with some old carpet, that's held in place with zip-ties. Oh, yes, and the shopping basket is attached to the rack also with zip-ties. Final part of the set-up is a short lead with carabiners on each each end..... one attaches to the basket and the other to Barnaby's harness.... correctly adjusted the short lead makes it quite difficult for a wriggly cocker to jump out ( but not impossible:-(
We used this set-up for many years with Betty who was a big old girl at 17kg. It works jsuta s well with you B who's still only 10Kg.
Hope this helps?