Mine have started nesting in the last week or so. I keep them in aviaries, so can't comment on the cabinet question.
Two nest sites is good, but make sure there is some screen or brush partially concealing it so they can feel a bit private.
When mine are feeding chicks, they eat heaps of green leaves and unripe seeds.
Some favorites are chickweed, milk thistle, dandelion, wild oats, lettuce (leaf and seed) and Phalaris.
I could post pictures if you aren't familiar with these weeds.
If a pair are slow to breed I give them heaps of green seed and leaf and they often get on with it.
Avid amateur aviculturalist; I keep mostly australian and foreign finches.
The art is long, the life so short; the critical moment is fleeting and experience can be misleading, judgement is difficult....... (Hippocrates)