For my V20 the "SKS Explorer" frame bag (
available on amazon) fits perfectly (but as other have said, your frame geometry is likely a bit different, so you'll need to measure it and then check the dimensions of each bag carefully):
View attachment 16976
The bag attaches in seconds with velcro and can easily be taken off. I use it to carry energy bars, as well as my phone, keys, and credit cards, so that I can take the whole bag when I go into a store. With some practice I learned to access the bag while riding, but that requires leaning forward while coasting. It is probably safer to stop first.