After changing /scripts/securetmp to
my $tmpdsksize = 2048000; # Must be larger than 250000

Use
umount -l /tmp
umount -l /var/tmp
rm -fv /usr/tmpDSK
/scripts/securetmp

You should see following response:
Would you like to secure /tmp & /var/tmp at boot time? (y/n) y
Would you like to secure /tmp & /var/tmp now? (y/n) y

Securing /tmp & /var/tmp
/tmp is already secure
/var/tmp is already secure
Checking fstab for entries ...Done
Logrotate TMPDIR already configured
Process Complete

PS: Try the following to see what large files are in /tmp:
ls -lahS /tmp/ | head