Not sure if they are a gadget, but probably worth having a go and reporting back here. If it doesn't work at least it still looks nice and is a talking point!
You've probably seen it but I just found this:
An excerpt:
You can help bumblebees by providing them with somewhere to nest. The first step of course is to provide lots of the right kinds of flowers in spring. At this time of year the nest-searching queens will be attracted to gardens where they can find plenty of food to help them produce their first batch of eggs. Once she is ready to lay, she will start looking for a nest site. She flies low over the ground in a zig-zag pattern, stopping to investigate holes in the ground, or piles of leaves. If you want to encourage bumblebees to nest in your garden, visit this page to find out more about our nest box trial