If you look carefully at my avatar pic in the bubble on the left of this post, you'll see my previous Assistance Dog, Max, weighing in at all if 2.5 kg dripping wet, sitting in an OEM RS Tank bag that's been modified to fit on the cover of the pre-2020 RT's, but AFAIK, there's nothing made specifically for the RT's, possibly because all the bag manufacturers think the RT's have enough storage anyway! I don't know if those tank bags for the GS/RS/RSS models are still available, but it was easy enough to put a couple of the 'hook' strips from a self adhesive velcro 'hook & fuzz' strip onto the glovebox lid so that it holds the base of the bag still, as it is very much the same as the fuzzy strip of the velcro; and the front securing strap went around the base of the steering shaft. However, once fitted, it was a right pain to get into the glovebox if I wanted to, but it did provide a very handy spot for my Assistance Dog to sit where he was safely harnessed in, out of the weather and in a warm and dry, wind protected spot that still allowed him to see everything. I fitted that tank bag back in 2014 or so, and while it's still around, I don't use it so much these days as Max passed on (at 18 years old) about 6 years ago, and my 'new' Assistance Dog is twice the size and weight, so he sits in a secured crate arrangement mounted on the pillion seat.
All that said, I've never used the clear plastic window thing on the lid of the tank bag to hold my Disabled Parking Permit; I simply slide the Permit down the inside of the Windscreen between the screen and the middle of the screen mounting bracket whenever I park. Originally, I used a discrete piece of tape across the bottom of the gap to stop the Permit sliding down and out, but the tape, which was not able to be seen unless you stuck your head down onto the frunk lid and looked up at the base of the screen/the very small gap, used to weather and need replacing every few months! So I found a clear plastic envelope/cover of the correct size to slip the Permit inside, a cover which has a flap closure on the top, and that flap stops the Permit in its cover from sliding down and out of the gap at bottom between the screen and the bracket. So it sits square in the middle bottom of the windscreen.
I can leave the permit there all the time, it hasn't ever blown out the top or slipped out the bottom, but I do tend to take it out and put it in the trunk when riding longer distances, simply so that it's not on show all the time I'm riding. But it stays there all the time I'm local running &/or even if I'm hitting the Freeway for the 40 odd km trip down out of the Hills and into the City. This spot is a very easy place to use, and a very good place to display the Permit, as once parked, it's far more visible to everyone than it would be if it was on the top of a tank bag, and because it's behind the windscreen, it looks like it's permanently affixed, so light fingers aren't tempted. I haven't lost one from there ever!
So, maybe you don't really need a tank bag after all??