Roll-up side ventilation.
Ventilation is most important in a Polytunnel not only in hot weather but also during most of the growing season. The addition of side ventilation gives protection from wind which you may get when the doors are opened, (and also butterflies and other unwanted flying insects), but allows air movement through the tunnel. Smaller Tunnels do not normally need this but longer tunnels will benefit greatly from controlled ventilation.
The addition of a gearbox driven side ventilation system makes for an easy way to open and close the sides using an extended winding handle. You will need to fit a Timber Base Rail and Side Vent Kit with this.