It's actually a sling bag that we bought from LL Bean
It had a strap to clip your keys onto in it and we just added another strap with a caribiner and that clips on his harness. He fits in nice. I also have a baby fleece blanket lining the bottom. Oz is not quiet 4 lbs..so he is a small little guy..but don't tell him that.
If you have a bigger Baby it may not be roomy enough..