I've been told that for a sufficiently large thermonuclear bomb, it isn't even necessary to compress the secondary -- just heating it is enough. Teller's original "Super" idea would work at scale. So once you've made a big enough bomb, attach it to a big tank of deuterium with thick walls and you're good.
The problem of making such a bomb comes down to the problem of economically producing deuterium.
The problem of making such a bomb comes down to the problem of economically producing deuterium.