I chose to solve teh problem by fixing the Varia to a seat mount, allowing more room for the bag.
https://www.amazon.com/gp/product/B088WZ31J4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I then picked this EVOC bag and it fits perfectly.
https://www.bikester.co.uk/evoc-seat-bag-m-orange-1068524.html