It is not a week job. Two days at the very most. With my limited skills, I believe this would be maybe 4 to 6 hours work.
You do not do it when you are sailing because fitting is complicated when the boat
is sailing. You do it back home and no sailing time is lost
Fasteners are cents.
And it does not have to be Sunbrella unless you are sailing a Mercedes and in this case the discussion is not relevant.
It is simply a choice between learning
something new and having fun building OR buying
the bloody thing. That simple.
600 per 45' is not a rip off if you buy it from any normal shoreside business in any civilized (unfortunately) country. I bet Ft Lauderdale and Stockholm this may actually be the lower limit.