A surface-mounted flange will not work. The railing must be fastened to a structural grade member, i.e. a foundation or footing. For this application, you should get a railing with long posts to be embedded into the ground with concrete. The block must be removed in the location of the rail posts, and dig holes for the post bases 18" deep. Set in concrete to the level at the bottom of the blocks. Cut the blocks neatly to fit around the post bases. Have the cut blocks ready to set in the concrete as soon as it is hard enough to support the blocks without sagging.

Remember the railing must withstand a force of 200 pounds at the top. This load transfers more than a thousand pounds of force on to a small base plate flange.