Hi Matto. I'm currently flicking through a book 'Aquarium Care of Bettas' by David E. Boruchowitz, which has this to say on the subject......
Quote..."The males bubblenest is a vital and distinctive part of the spawning process but males vary greatly on the details........Some nests are loose and scattered, while others are dense, rising 2.5cm or more out of the water. Large nests may almost cover the surface of the tank, while small ones may be tucked away tightly in a corner. Some males will build virtually no nest at all and still be successful........Many breeders find that something floating will soon have a nest built under it...........University investigations have shown that males prefer something that is round, about the size of a Grapefruit, and yellow in colour.......these characteristics correspond with dead leaves"....End quote
HTH