On my 30L speidel, the stopper didn't seem to be snug either. I'd push it down and it'd slowly ease itself back up. I'll have to try the dry twisting method that HopSong mentions.
I noticed that geer537 has his different than what's on the
morebeer picture. I had mine the other way, with the plastic threaded piece pointed up.
I also noticed that I had a lot of extra room with 5 gallons. I plan on scaling the recipes larger for it, but am here looking to see if I should be scaling for 7 gallons, since it's advertised to hold 7.9 gallons.