On Sun, 03 Jun 2018 14:44:02 GMT, Frank
m wrote:

I am trying to install a ceiling fan (about 17lb). The outlet box was plastic
and probably not rated for ceiling fan so I have it knocked out and was
planning to use a ceiling brace kit. However I noticed that the joists was
too close apart and the bar of the brace kit is too long. One joist is
flushed to the edge of the hole. I wonder if I can just screw a metal outlet
box to it. Will that be strong enough as the support is going to be only on
one side of the outlet box.

Use construction adhesive to add a block to the side of the joist and
screw the box to the joist and the added block.
--
Mr.E