WEST BIKING Fahrrad-Oberrohrtasche, magnetisch, wasserdicht, Schrauben, Riemen, 2-in-1-Fahrrad-Fronttasche, MTB, Straße, Kies, Radfahren, obere Rahmentasche

I've tried numerous top tube bags and the best I've found is this one which is 1L and only 5cm wide. It's got a strap or bolt on option and has a sturdy magnetic clip. It's seems to be a common model which is produced under many brands - I think West bike was the cheapest for me from the coins page. There's a couple of bolt mounting positions and the forward position will minimise the gap which is fine for me, but not sure if it's acceptable to you.