I like these sides.  I think the one on the left will be a bit easier to make, but the one on the right does have some additional charm. I might be able to make the trim with a thicker plastic, but I’m not sure about the interior detail.

This seat back has a bit more character. A simplified end could be built fairly easily.  Actually, it might be better than the side above, but I wouldn’t add the detail – the double lines and circles on the armrest.

I’d cast the sides separately from the back and the cushion on all of these. Or, maybe it would be best to cast the cushions all as one piece?

This is one possibility. I would recommend a plain cover, though!   This could be done in 4 pieces, perhaps?  (2 sides, a back, and a cushion.) The legs would be cast into each side.

Assembly could be problematic just because there is not much to fit against..

These two are pretty similar.  I’m not sure how easy the tufts on the one on the left would be. (Heck, I’m not sure how tough any of these really are!) I’m thinking that the upper piece (arms and back) being cast as one piece, and the cushion as another. I guess the base would be the third - though I’d love to find something cheap that we could use as the base!