I'm a semi-regular Fortnite player, but I've never once tried to return to Spawn Island. A quick Google search informs me a sizeable chunk of players have made the trip by building bridges, parachuting straight from the Battle Bus, or hitching a ride on a rocket.
Invariably, these methods—the ones I've seen, at least—have fallen short due to the invisible wall that guards the archipelago. As showcased by Fortnite streamer Lachlan, though, it appears doing so while riding a Shopping Cart is possible.
As shown from around the ten minute mark there, the pair design a ramp platform that sends them soaring through the invisible wall and onto Spawn Island. After first making the jump, they appear to momentarily idle in a sea of blue, before emerging at the other end directly above their target. They expire seconds after touchdown.
"We were flying," says Lachlan towards the video's end, "and then it just stopped. And then we were falling. And then, there we were, Spawn Island. Look at that, we made it boys. We died almost five seconds later, but, that's what matters: we made it."
Lachlan then points out that the shopping cart also made to Spawn Island intact, and ponders whether or not trips back and forth might be possible.