Fitting the cowlings

The two cowling halves need to be trimmed to fit each other and the plane. The manual suggests clecoing them together and adjusting to the firewall. Sounds reasonable enoough, but you dont know how to put them together UNTIL you know how they fit around the firewall, and you can't find that out because the propeller hole won't fit around the flange at this stage. Fitting the top first and then the bottom is also not an option because the flanges are the wrong way round to do that.
The only option is to do it in exactly the "wrong" order and start with the bottom cowling, despite having to work against gravity.