We have a couple of the DHB bags, they are huge! All I do is drop the saddle a few inches, use a releasable zip tie to hold the mech forwards against the chainstay so it doesn't get bent, and then throw the bike it which ever way up is most convenient. I've got a medium Yeti 575 with wide bars. What car did you say you were getting? The bags are a squeeze in the car, we had a Mondeo estate which is huge, considerably bigger than the Octavia, and they take up the whole of the load space. And you will still get mud in the car, inevitable when you have to put a muddy bike in the bag in a wet car park.
Plus if you keep the bikes indoors, you will end up scrapping all your walls and door frames as you carry the bag out!
Just get the towbar fitted, easiest and cheapest option.
Edit…just noticed from your old thread you are getting a Fiesta? I don't think there is much chance of getting the DHB bag in there full stop.