If so inclined, you may want to try this:
I would think that if it can hold fuel, then surely it can hold air.
If not that, there’s a couple of products I only just recenty learned about which are no doubt locally available to you:
bondo plastic metal
and
bondo glazing
I learned about it from this guy:
Don’t worry. Despite appearances, it’s not some kind of S&M contraption. 