Ok here's the way I mounted the Dinotte tail light. Instead of getting a bigger seatbag which I didn't really want I bought a length of velcro from a crafts store and sewed it to the end of the seat bag closest to the seat post. Use the thickest thread you can find and make sure you use a very sharp and thick needle as it's kind of difficult to punch through the fabric otherwise.

After sewing the loop on I simply used an extra piece of velcro to attach the loop on the back of the battery pack to the loop I sewed to the bag. I then used the Dinotte strap to wrap all the way around the battery pack to protect it against additional bumps.

The Novara seat bag isn't waterproof so I didn't bother using seam sealer but you can see that it's out of the weather for the most part anyway as it's right under the seat.

I used a piece of washer hose and some zip ties to attach the Dinotte light to the existing loop on the seat bag. I can adjust the level of the tail light by pulling the O-ring towards the front or back of the light. Currently I have the light pointed somewhat down so it projects more towards the ground than straight back.