May I ask why you'd want the mixer on the floor and not at waist-height or higher where it'd actually be accessible? I use a simple 4-channel mono mixer and I find I'm adjusting it fairly frequently while playing live to tailor my levels, both to keep a balance between sounds (I play mainly at church and, unfortunately, we don't rehearse other than Sunday mornings for an hour, where I don't have all kinds of time to get my sound balance just right on my own, let alone with the rest of the band) and because balancing for the acoustics of the room or to compliment the other musicians is something I try to do. Same deal with organ drawbars; I'll make a preset ahead of time but do small adjustments as needed based on what I hear and what others are doing.
If you were to use a case, I'd think that anything that fits your mixer securely and is durable enough would be fine. You'd still have to plug the cables into the mixer though (or, at very least, plug them into the keyboard); I'd think it wouldn't be much harder to just deal with it manually and not bother with a case. I use 5 pedals, two keyboards, my amp, and the mixer, and can get everything set up in under 15 minutes.