I'm sure trailer leaf springs would work, but I think that they're pretty narrow and you'll have to make up for that some how. Like using some grade 8 washers to make up for their narrow width.
Really, you could use any leaf springs that would fit under the car. Just keep in mind that each leaf spring has a different spring rate which you'll want to consider. You probably don't want anything with too high of a spring rate or your rear will be hopping off the ground when you run over a bump at higher speeds.
I think you're on the right track with the Samari springs cause they're made for a light vehicle and the body isn't really long after the wheel wells and neither is a beetles. Whatever springs you decide on be sure to get the springs and mock them up before you shorten the chassis so that way you'll have the correct wheelbase.