I need to put some risers and a manhole on top of a plastic septic tank so I can pour concrete on top.  It will be a low traffic area with the heaviest loads being a skid loader and 3/4 ton pickup. The tank was set on a gravel base and has gravel all around it and I'll pour 6-7" of concrete on top of it with rebar on 12" centers.  I need to put a riser on one of the clean outs and put a manhole in the concrete to access it.  What is the best way to do this?  Its a snyder tank that is about 10 years old and we were able to move it for a building addition.