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.

Yes you can, here is one type of box designed specifically for the
job.

https://www.homedepot.com/p/Madison-...BFAN/203343431

If you use a regular box, don't use the black drywall screws to attach
it, use deck screws, they are much stronger and less brittle.