As glass tends towards 7mm at full fuse, it is difficult to keep straight edges on thick pieces as the glass moves. If you want straight edges without dams, there are a few solutions:
1) Don't flat-fuse - apply less heat work so that the stack stays vertical instead of spreading. The degree of tack fuse required will be a subject of observation and experimentation.
2) Plan on trimming the edges straight. You can use a saw or grinder and then either cold work the edges to polish, or fire polish.
3) Add a couple of centimetres or so on each side of your base glass, so that a 20x20cm piece becomes 24x24cm, and flat-fuse as normal. The volume change will (mostly) be absorbed by the extra glass, so that you can simply trim it back to the right size and cold work the cut edges.