You know the drill, pics all the way through please.
If I remember correctly from an old pic in here you're about 5'8"-5'10" and 70-80kg?? If so you'll get away with less float than me and it'll also determine the planshape too, as you'll do best with less foam in the tail of your board than I can being 6' and 105+kg. And you want a tube machine yeah? So you really want to keep the area down in the tail so the foamball doesn't pick you up and turn you sideways, that never ends well.
Now this can translate into either an area pin thruster or a twin/quad fish and they are two very different beasts and involve totally different styles of surfing. The modern kneelo scene is built around the fins forward thruster ala Flashpoint or Parkes, as so ably demonstrated below by a 50 odd year old 4 time world champ;
#!
His boards are shaped by Bruce Hart in Sydney, I've had two of Bruce's boards and absolutely hated both of them. You'll note in the vid how Sparra is leaning so far forward, this explains the whole fins forward thing(pivot point) and whilst I understand how it works, I've just never been able to get it to work for me. And I don't think it's just a fat thing as a bloke I know that is built like Sponge Bob with rounded edges rides Bruce's boards and he absolutely shreds.
The other school of kneeboarding(and modern surfing for that matter!) came about via the Steve Lis Fish. Fins back, a foot or so shorter than fins forward boards and ridden off the tail. I model my boards off the Lis platform but have played around with the centrepoint, concaves and generally filled the swallow tail into a diamond. I also like my fins somewhere between front and back.
The top board in this pic is very Lis, so is the bottom one for that matter but I prefer the quad...
http://www.ksusa.org/Forum/album_showpage.php?pic_id=1218and I reckon something like this would suit you to a T as a tube machine. Also more flexible to use as a standup fish too, they're one and the same after all.
One thing I reckon you really need to address with the Lis shape is the hull entry thing. I reckon it's a dog, it makes paddling so damn hard, you're pushing water the whole time. Instead go the full on(8-12mm) single concave in the nose into a double through the guts, shallowing out through the fins and flat out the tail for release. Also allows you to do away with hard rails so far forward which allows for more margin for error. Much the same effect as Arkgee's chines.
If you're not going to use flippers then I'd go a 5'8" to 5'10" x 23 x 2.5" rail centric quad Lis-ish fish. If you're going to use it exclusively as a kneelo and wear flippers then you could go 5'2" to5'6" x 22.5" x 2" make sure you go a heavy glass job or alternately compsand as you tend to get worked quite a bit and it's always in the nastiest part of the wave.
This is my current fave and these fins are now avaiable in FCS tabs too. Tried this thing with lots and lots of fin combinations and keep coming back to these. This thing is almost a quiver killer for me except the diamond get's me into strife on the foamball and I get eaten.
One of these with a Dan Tomo- ish swallow will complete me.