I liked the sag but ended up not in love with the bag, so I sent it back. But I think other members have used a magazine to keep the bottom flat. You might be able to get a heavy placemat in an acrylic-type material or something and cut it to fit.
On most of my "saggy-type" bags, I cut out a bottom with my LV box. I just make sure to "round" the edges.
Although lately I've been doing that to some of my non-saggy bags. I think it also helps keep the bottom of my bags cleaner. I still won't carry any pens in my bags tho'.
I just use my planner and it works fine. I don't need to add any extra weight and it serves both functions. The other thing is, I just try not to carry so much and the bottom is fine. I looooooove this bag.
Hi. I have a 1 year old daughter and I use her board books. The book is about 1/4 inch thick and is about 6x9 (in inches) so it doesn't fit perfectly but it does the job. I place it in the center of the bag. It's sturdy and doesn't sag at all. I also use another board book for my Gucci 'Joy' Boston bag (similar to a Speedy 30).