New upvc porch - fittings and ideas

The point?

Our present small fence (to stop people driving into the ditch in front of the property) is made with pressure treated uprights. Unfortunately, they are not always visible (under trees) to delivery drivers, who have managed to hit them, or at least the horizontal timbers they support. This has ressulted in the main uprights breaking - they neeed to be replaced. I wanted to make the uprights a bit more visible - I can't really paint pressure teated timber.

It's likely to be too thin to take a fixing, considering the stress that a washing line or similar structure can impose. I suppose a butterfly-type fixing might be OK, but still likely to distort the steel section

You might do better just to attach some conspicuity aid such as the standard yellow and black reflective film on an aluminium base, with which the drivers will be well acquainted. Or as I already wrote - galvanised steel box section. :-)