how to logrotate /var/log/btmp?

So, I got /var/tmp/btmp filling up on one of my NASes (running latest OS6). I know why (so don't start off by telling me something bad is happening, I have fixed that)./

What was concerning was that the file was allowed to grow (apparently) without bound on my limited root file system. I added an entry to rotatelogs.d but couldn't even tell if that was the proper mechanism.

Here is what I created:

/var/log/btmp {
monthly
size 100M
create 0600 root utmp
rotate 2
}