Best way to fill moderate gaps in greenhouse

I've (finally) finished getting my greenhouse reinforced against strong winds and am close to the point of being able to start growing things in it. I have noticed that on the sloping roof sides, there is a gap between the roof panels and the wall panels, of about a few mm, maybe a little more. It is not due to incorrect installation, it is just the way the frame is built. Since I am planning to start using it in April, it is unlikely to be an issue (and may be beneficial if it is warm), but next year, if I want to try starting things in February or early March, then lack of heat retention will be a problem. What do you think is a good way to fill long gaps of this size?