I had the Shoft on my Volvo C30. It was OK at best. It really only works if the seatbelt receiver is low enough-- otherwise the rubber brake won't be pressing against the lap belt.
These days I'm using an Infasecure Lockie https://infagroup.com.au/collections/universal-hardware/products/lockie
and it is waaay better than the Shoft. It's a thin wedge that presses into the buckle where the belt passes through. Pull the lapbelt tight and press the Lockie in while maintaining the tension. The lapbelt's tension pulls the Lockie in further, enhancing the grip. It works pretty well.
But it's not perfect. Some drawbacks:
-It's a bit bulky, and if it gets bumped it can be easily dislodged.
-If you forget it's there when you remove the seatbelt there's no telling where it'll end up. I bought two initially and I lost the first one within a week.
-If, out of habit, you pull on the shoulder belt to tighten the lap belt without also pressing the Lockie into the buckle, then the Lockie will end up on the floor.
-Doesn't appear to be sold outside of Australia. When I ordered two Lockies shipped to Germany customs was nearly as much as the Lockies.
Still, it's the best thing I've found and I'll be getting a couple more as backups. I might also remove some of the excess bulk and/or tether it to the seatbelt receiver.