You will get both smoothness and shape right as homebuilder as long as you have enough sanding paper, filler and patients as thousands of homebuilder and model aircraft builder have already shown.
To get the section shape right you can work with a negative template gauge (don't know the right english word for it, a plank where you cut out the sape of section). With that you can easily see where to remove or add material. You have to sand and fill until you have the shape. The section is more sensible to errors at the nose than at the end and more sensitive for errors in streamwise direction than along the span. I hate sanding but I go down to 1000 and then start polishing, but if you after a nice look as well you may even uses finer sanding paper.
The real problem is that a catamaran has 4 foils... but after sanding to hulls you are already into the zen of sanding.

Cheers,

Klaus