Check the weather forecast and hope for rain? Set up some wicks with buckets of water that will suck water into the boxes if they get dry? No experience with this myself but read about it on the forums. Set up a couple of buckets with drip lines (aquarium tubing) and clamps (clothespins? Bag clips?) like IV drips? I’d ask a neighbor to water but you said other than finding someone to water. If you were near me I’d do it.

Can you run a garden hose across the window boxes and make a few small holes in it over each window box so it acts like a soaker hose and put it on a timer at the faucet? Timers are pretty cheap. You’ll be trashing a hose but you can probably get a cheap one and use it later as a soaker hose somewhere.

Soaker hose and timer. Your local plant nursery/greenhouse

Screw the timer into the spigot and lay the soaker hose in the planter.

Unless there is a lot of space between the window boxes. That’s why I suggested a hose instead.