When I built mine I had access to a heavy steel workbench (welding table), many clamps and scrap steel pieces.
On my kit the left side plate was already attached, I clamped it down, and rough fit the right side plate. I used an 80% RSP, drilled the remaining holes for the rivets and got to work.
I made up bucking bars for behind the rivets and for riveting I used an old cold chisel turned down in a lathe to give a concave "pretty" rounded finish to the rivets. I used a large ball peen and constantly checking that the rivets were held tight I hammered the bejeezus out of them. It didn't take much effort at all to "smush" the rivets into shape.