Either Secret Agent, or another pickguard.
Really any pickup could work if it fits under the guard. I think the secret agent is lower profile and you’d just put in some foam to keep it from rattling around.
It is also much higher output, to try and make up for the fact that it is so far from the strings. Though, that only helps a little.
Has anyone put in a standard tele neck pickup in an Esquire cavity under the guard?
The result is a significant difference in both volume and tone between the neck and bridge pickup. If you want an idea of what it will sound like, take any guitar where you can adjust the height of the pickups, and lower the neck pickup down to below the level of the pickguard/wood (if you can even get it that low). Then play the guitar through your amp, and switch between the bridge and neck pickups. The neck will sound very weak and lifeless compared to the bridge. Unfortunately, that's just physics. They improved the issue a little with the Secret Agent, but it's still a pretty big difference.
If you scour the demos, you can find many examples of the Secret Agent sounding great. But, if you pay attention, they dialed in their sound for that pickup. Notice they never switch to the bridge pickup, or go back and forth. You can make the Secret Agent sound good if you dial in your tone for it, but then the bridge will sound super hot/loud in comparison. You can get a compressor to boost the neck and squish the bridge a little to balance them a little more. But, then you have a compressed bridge tone, which you may not want. Or, you can add a boost to your signal chain that you only engage when you're in the neck position. The point is, it will take work to dial things in so both sounds are usable together, switching back and forth.
I vote for an alternative pickguard with both pickups. Reversible for re-sale value, and will ultimately sound better. You lose the "look" of the Esquire, but gain the tone you're looking for without significant effort.