not sure about the Velcro, it was wanting the lights that started this given my current bag on the seatpost rather prevents, the velcro would attach but I think the whole thing would lack rigidity ? and lights may not always point directly backwards, certainly seen riders in London make this mistake at night with lights on backpacks, this is about being seen rather than complying with law at night from my perspective. I think the easiest is the "Pro" bottle solution leaving the seatpost to be "light-tastic", do like the "mini" bag but more expensive for less space.
thanks for all the comments everyone
thanks for all the comments everyone