The floor board would have the advantage of protecting you from road splash. but after indenting from pedals, which would seem to be necessary, you'd lose a lot of that.
And it would be pretty heavy, I suppose. but I imagine it could be done.
If you want it at all, then I'd give it a try. You might come up with something neat.
And a side-car is possible. There's someone in this forum who made one. I'm afraid I don't remember who, but you ought to be able to find him by searching.