If you look at the bags linked in the first post you'll see Tim is looking to drybag type roll bags that are used for bikepacking. His concern is the ends interfering with the hand positions.
Some designs crowd the drop position area by butting up against the bars but this is just having too wide a drybag or too narrow of dropbars. Flared bars help but if it is too crowded you have a hard time pushing your thumbs through to use the brake levers. That can be solved mostly by not overpacking the bag and rolling it tighter. I also smush it around once on the bike.
Most commercial products clear the flats area by having the drybag mount proud from the bars via the attachment system. That can be a problem with the DIY solution I use but again, I just smush the bag around and fold it in a way that works out. I mostly ride hoods or drops now so it doesn't matter that much.
In this pic one can see that I have just a little (but enough) clearance in the drops and the other shows the red retaining strap that came with the Blackburn system.
