I need an idea of how to build a 15" manhole cover from steel?

1) Use Google - select images - enter manhole covers - get some ideas

2) Check out this and other similar sites...
http://www.fjones.com/construction/construction-products/drainage-products/manhole-covers/

3) Visit your local builders merchants and check with them what they have on the shelf

4) Ask yourself questions about how you might cut the steel plate (who you might get to do it for you) and ask yourself if the thickness you mention is really appropriate for the cover plate.

Good luck...

Peccavi
July 2013