I have had a Bagster tank cover and tank bag on my Ducati ST4S for probably 15 or more years, good quality. A downside of many tank bags is that you have to take them off to access to petrol filler caps. The Bagster gets around this in a convenient by having two spring-loaded catches at the back that when released allow the bag to be pivoted forward on the two front fixings. The two front-fixings are fairly quick to release should I want to remove the bag completely leaving the tank cover in place.
I also have an inexpensive magnetic tank bag (purchased ages ago at Lidl) that I used on my Rapide. Cheap and cheerful but very convenient however when I fitted an aluminium tank it was no longer convenient as it had to be held on with an inconvenient selection of straps.
Peter's mention of a Bagster Harness and tank Bag suggested that could also be a solution for me and enable me to use the tank bag from my Ducati. A bit of research found two places that sold the Bagster EasyRoadEVO harness. I ordered one from BargainBikerBrands on Thursday and it came free post post yesterday. I had a quick trial fit last evening and it looks as if it is all going to fit. (OK, so the Ducati bag is red but I like red
.) I've posted a couple of photographs below showing the harness in place. This aluminium tank has a central filler hole so this harness works for me. It could also be a good starting point for someone that was prepared to modify one to give access to the standard tank filler cap.