I have had a few requests for this page and had to repost it since the free web space I had has disappered without warning. It has been about 3 years since I first made this page and the conversion is working fine.
Here are pics of the Suomi to M11/9 Magazine converstion done by a Class 2 manfacturer that is a friend of mine. My M11 seems to be used as the protype for all his ideas. So far they have all worked. If you have access to a milling machine and are good at using it you should be able to do the conversion. If not Find someone who is.
Ok, here is what was done. The picture below is of the upper. Note you have to open it up to make the trapezoid mag fit into the upper.
Below is the Bolt, you have to mill off the bottom (center section) so you have magazine clearance. With the double stack mag the bolt will hit the mag with no chance of it ever working without this mod.
You might notice the screw head (actually that is the bottom of it, the line is where it rides on the bolt) where the original cocking knob goes, we had to do that because with the bolt milled off the front section where the disconnecter engages falls down in the slot where the disconnector protrudes which ended up binding the bolt. Just make sure the screw barely rides on the barrel when the bolt closes so it won't fall. (that is the only draw back we found which over a long time might wear a grove in the top of the barrel).
Below is an unmodified mag and PICS of a modified one. You have to remove the front plate from the original and do a light weld to hold the seam together. Mill off the back of the mag till it fits in the mag well. Then we used some sheet metal to extend the back so the mag catch will engage using epoxy to attach it, we might weld it in the future. We also added a mag stop on the front so you don't have to do the insert the mag and pull down bit. the biggest thing with the mags, is getting it to lock in place in the right spot, if it sits to low it won't feed right and to high it might hit the bolt or not feed right, we got lucky and didn't have one sitting to high yet.
I hope all this helped, It really seems to be a great idea and so far it has not let me down. I have 4 mags converted now, I am waiting for 9 more to get back from being welded so we can add the sheet metal to them. I used 2 mags (that's all I had at the time) last month at a match and they ran like a dream. Having those few extra rounds in the mag was nice. Even with the TASK type conversion we have done to it, I can load all 36 rounds into the mag and it will still strip the first round. Something the other mags I had didn't do so they had to be 1-2 rounds short to start. Also you can still use the factory original mags in the gun.