Both Socom MFG and Kurt's Kustom Firearms make a block for about $65 that use sten mags. (check Tapco.com for cheap sten mags) A lot cheaper than a Colt/ASA/RRA block that sells for twice as much. But like was said before, you can't use the regular mag release.
Socom Sten blockKKF Sten block If you really want to use the regular mag release, you can also find modified sten mags that don't require a block with the paddle release on the bottom. They come up now and then on the EE board. I believe that Olyarms will modify a customer supplied sten mag for about $25. I believe the mod uses screws so you can unmodify it to reuse in the original sten (as per AWB hi cap rules i think -sorry, but I'm not an expert)
If you want to have your sten mags modified, I would buy a socom or kkf block and a bunch of sten mags. Test them to find the best in the group. Then sell off the block and send the mags in to be modified. That way you know that you'll have good funtioning mags before you spend money to alter them.
tspike has some mags at a pretty good price, if they're still available.
modified sten mags