My base isn't slouching at all; now that the leather has softened the sides are pooling down and unless the bag is more filled the sides cave in. I really like this bag, but I too like its original rounded shape. I don't think a base shaper would help and I've been really happy with the quality of my Samorga organizers (I have them in my LV Neverfulls and my Rogue Tote) so I decided to have an organizer made for this and for a bucket bag I love from Leo & Violette. That one isn't slouching at all, but I would like inner pockets to make finding things faster.
I ordered one of the LV Speedy 25 styles (LV-S25-3) for the Ace, but adjusted the dimensions. I've asked for L10", D6" and H4". Because the bag narrows so much at the top I didn't want to make the dimensions quite as large as the base or have it come up too high because then it would collapse inward when zipped, which would defeat the purpose. Hopefully I judged things correctly, but I think it will be fine.
I'm pretty sure you can order an organizer without any inner pockets if you don't need them, from Samorga or others.