Two years ago I bought (payed shipping, really) I Sevylor with a reinforced floor from a friend of a friend in AK. It floated, well enough to act as a proof of concept and confirm that I would use and enjoy a proper packraft.
That said, an Alpacka really is another category of boat entirely. The dinky tubes at the small end of the Sevylor mean that you get wet from even the smallest waves, and also causes the boat to paddle poorly. The reinforced floor is fairly necessary, as the Sevylor fabric is crazy fragile, but the vinyl and glue makes the boat heavy, bulky, and hard to pack. Much moreso than an Alpacks.
Phil Turner's video of the Flytepacker is very informative. It seems like a solid option for lakes and calm rivers. The video also makes it obvious that Alpackas are in a different league as far as design and construction. I had high expectations for my Alpacka, and it has exceeded them by quite a bit.