1. Is the LA3 lens adaptor made of plastic or metal? Is there an
aftermarket supplier of a better one? What is the filter-thread size
on this adaptor?
The LA3 is metal. The fz18/28 lens extends about 3/4" from 1x-18x. The fixed length LA3 will cause vignetting below 2x zoom. The anodized aluminum Pemaraal PA62E adjustable adapter allows for use through-out the zoom. See http://www.pemaraal.com/index.php?page=fz18ac .
Check 'fz18 adapter' on ebay for other options.