Thanks for the eIO spawner and diamond spikes tip. I'll give that a go!
I haven't had any trouble with wither escaping the cage over the 20 times I've summoned him, but I was careful to make the cage tall enough that he has room to float up after his power-up process completes (this required looting two volcanoes to acquire enough wither-proof glass). I recall messing around in creative ages ago in vanilla, testing techniques for caging wither using obsidian (didn't work, by the way--he'd chew through the obsidian almost as fast as he chews through other blocks). I had found that if the cage wasn't tall enough, he'd float right up through the top after his initial post-power-up float. But perhaps I should move my wither cage a bit further away, just in case!