Learn English – Using the prefix “be-“

The Online Etymology Dictionary reports the following:

early 13c., amasian "stupefy, make crazy," from a-, probably used here as an intensive prefix, + -masian, related to maze (q.v.). Sense of "overwhelm with wonder" is from 1580s.

So the a- is a prefix here, but it intensifies instead of negating. Wiktionary lists the several uses of a- and notes the Greek-based meaning you cite as the only one that remains productive today.

Depending on context quasi could work. Quasi as a prefix indicates similarity or resemblance. If a sphere is a ball and a hemisphere is half a ball, quasi-sphere would mean something that's ball-like, "roughly" or "more or less" a ball.