This might take time, but start building up now and it could come in handy for next year’s heatwave.
Sit up tall away from the back of the seat when on public transport. It lets the air circulate round your back, and works your core muscles. If you’re not used to using your core you will want to slump pretty quickly. That’s ok. Keep practising and the muscles will gradually strengthen.
It really only works when the bus or train is actually moving, and I haven’t tried it in rush hour… but worth a try?