I don't have a specific suggestion, but some of the flat wireless chargers may be small enough to be considered portable.
Another option may be a battery powered wireless charger.
The most portable option will still likely be a wired charger. I'm usually charging multiple devices when I travel so I use an Anker 5 port charger that has one quick charge port. But I'm usually traveling with someone and this allows her not to have to bring chargers either or I'm traveling for work, which may require me to charge some tablets so it works pretty well. I doubt I'll ever bring a wireless charger in addition to it. Just too much clutter.