You need to change the tube ID so that an original bolt will not fit. You could possibly use the original tube ID and add a couple of guide rails that would prevent the installation of the original bolt, but that's a BATF decision area.
You would need to convert the rifle to closed bolt firing. IIRC, the Semiautosten.com kit used a striker that was held by the original sear while the bolt continued forward to strip and chamber a round. The striker would be released and propelled down a pair of guide rods to make contact with the firing pin. You'd need a custom bolt with a firing pin (firing pin spring wouldn't be a bad idea either) so you could make it for a vertical mag while you're designing. The bolt would have a flat bottom that clears the disconnector and sear. The striker would take up the entire inside diameter and trip the disconnector. This would allow the striker to catch and hold on the sear. You could make up a firing pin retainer similar to that on a 1911 slide.
A spring setup similar to the M11/9 recoil arrangement would be a great idea for the bolt. A second set of springs/guide rods would be used to propel the striker. The guide rails would keep things running true.
I think the striker and bolt made contact with the entire inside diameter of the tube. This would ensure that the disconnector was tripped regardless of the position of the selector. I seem to recall that there was some type of mod to the selector to keep it centered (in the semi-auto position). Again a BATF area, but I would think a block welded into the trigger housing would suffice.
Add a 16"+ barrel to this rig and you're set.
Before you do ANY of this, I would call the BATF technical branch and ask for a written set of documentation. They have already made rulings as to what is acceptable and what will get you some prison time.
With a 16"+ barrel, I think a fake suppressor can would make the rifle look much better. The skinny barrel on the semiautosten.com version looks odd.
the barrel on the semiauto sten kit was an IMI barrel, it wouldn't look to bad if the tube was left longer to cover up some of the barrel. There is also a sten group at yahoo groups that discusses about the semiauto aspects. There was the semiautosten.com and also the renn design. I am sure there are others but these are the two that I recall (other than the catco design for the MKII - which interestingly the semiautosten kit is designed similar too).