If you feel your E string is too floppy, maybe you need to get bigger strings. Judging by the slots on your nut, it looks like this bass probably came with some thick strings.
If you like the flatwound sound and feel, there are a few alternatives you can consider:
D'Addarios ECB81S Short Scale strings - they are a bit thicker than these Rotosounds, and on certain basses, they really give it an optimum sound. But it totally depends on the instrument, IMHO.
The DR Hi-Beam flats, that are the same gauge - 45s to 100s.
Labellas make some that are fairly stiff, but they are pricey. They also sound considerably darker than the Rotosounds you have on right now.
The Thomastik Infeld's short scale strings are a little thicker, but they're pretty floppy by comparison.
If you can get over the initial tacky feeling of the GHS Brite Flats, they're probably the thickest strings you can get for a reasonable price. It's a love-hate thing with them, though. Once the tackiness is gone, they do sound pretty good -- they are similar in sound to the Rotosounds you have on. I did try them on my P-bass, but the G-string broke, and I decided to go with Pressurewounds for fits and giggles.
All of these strings can be bought at bassstringsonline.com or juststings.com