Abs grating is 2 3/4inches thick very strong and flexible so yes you do need to support however you have to support steel grating as well.. i always custom build my basins with liner and grating. This way it gives me the ability to place an access panel where ever I want for pumps. If you take time to break down costs its actually cheaper my way than buying those expensive basin kits but I'm not limited to there sizes and therefore not worrying about how to get my pump etc. I posted a thread a couple of days of a current job and one of the pics shows the grating.
Turner Landscaping, LLC