OK, I think everyone's convinced me not to try custom construction --
thanks.

What's the reputation of Sun Tek? I have not yet consulted local dealers,
but I see those as an option on the big box web sites. Somewhat cheaper
than Velux though not dramatically so, actually seem to have more useful
info on their web site. And they offer a number of models in polycarbonate,
as well as tempered glass.

I know that whatever I use, it will have to be curb mounted. AFAIK, this is
required for all skylights on metal roofs.

Solatubes would be great for interior lighting. But no matter what optics
they use, they can't pass through more light than they collect, and the
main models collect from one or two square feet of roof. This is plenty for
inside, but since I'm talking porch, I'm interested in more like 20 square
feet. Velux and Sun Tek both make skylights in nominal 2'x'4 sizes ...
well, two of those would be only about 15 sq ft, that might be enough, or
add a third. (And my roof is mostly shaded.)

I also need to call the fabricator to see what they recommend. The
responders on the forums at metalroofing.com always harp on "consult the
manufacturer".

Thanks,

Edward
--
Art Works by Melynda Reid: http://paleo.org