Proper tip specs:
5.5mm OD
2.5mm ID (I think it's actually 2.1, but I have a 2.5 that works)
12mm in length or longer (all of the delivered tekkeon plugs are 10mm)
Tekkeon makes a special plug that will work but it's not sold with the batteries, it's for a Panasonic laptop, part number
PA-S5F on their site. My existing plug which is ok is that spec so per tekkeon support, that should be fine (I haven't had my 12mm L/2.5mm ID tip fall out or produce noise when rotated which the 10mm length plugs seem to). I put in to support to see if they had a 12mm length w/ 2.1mm ID and I'll edit this post when they get back to me (last reply was at 12:30AM on a weekend, kudos).
If you're using the 16/19v powerizer laptop dvd batteries, you're already using a 2.5mm ID/12mm length tip (its the only one I could get to work with mine).
edit: they don't have a 2.1, but did say you can shave the black plastic some to achieve it. The 2.5 that is linked above works (arrived today) and is solid enough for my own comfort, it won't fall out by any means but I wouldn't want to try and hang the sax by it level of solid.