This is a very late response to your question, but perhaps it will help others.
I bought the Topeak Omni Quicktrack adaptor, which allows me to use the Topeak quick-release trunk bags. It works very well. I've seen some posts saying that it has spontaneously released the bag on some folks, but I have not experienced that myself, and I ride over some bumpy surfaces. However, note that I haven't used the elephant-ear paniers with any significant weight yet. I will say that the location of the water-bottle holder on the rear of the trunk bag is not ideal -- I've had water bottles pop out twice so far. One was spectacularly exploded by a following Prius.
This adapter was (in early 2021) very hard to find. Finally got one in May from blueskycycling.com.
HOWEVER, mine is mounted on my Medeo T10+ (photo below) -- not a T9. And it was a bit hard to get the underside brackets to clear the stock fenders on the bike. I wanted to mount the adaptor farther forward on my rack, but clearances would not allow it. As it was, I was worried that I was distorting this (plastic) plate in a way that would make it hard to slide the trunk bag on/off. That turned out to not be a problem. The bag slides on and off very easily.
Brackets under the rack rails. The diameter of these Gazelle rack rails is near the outer limit of what this adaptor can handle.
I have NOT attempted to mount this adaptor on my wife's Medeo T9 (photo below). Just from looking closely at it, it does appear that the brackets MIGHT clear the battery -- but it will be VERY TIGHT, at best. If you are still interested, I will do a fit test. ( I would just go ahead and do it, but it was difficult to get the rack on my T10, so I'm not looking forward to having to redo that.
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