What’s a wood magnet? It’s a level that press-fits onto a 2×4 to make one hell of a long beam level. Assuming you can find an un-bowed and un-warped 2×4, it sounds like a decent idea. Of course, that’s a pretty big if.
The simple horseshoe design sports three bubble vials each measuring on a separate axis.
I can’t imagine that this thing is going to be the most accurate instrument on the jobsite, but consider this: it’ll cost you about a quarter what a standard 4’ beam level would, even with the cost of an 8’ 2×4 thrown in. And you’re also quite a bit less likely to break the part that matters. So if you can live with the accuracy, it might be useful.