The closing panel can help keep the pump firmly planted.
Once you get everything in place and tight. Put the closing panel on. The panel won't sit correctly. So you start the first few threads on each screw all the way around then you can start tightening them all up. Then there will be pressure on the pump and it won't move. You shouldn't remove the springs because that keeps the pump at the bottom of the tank.
People have cut holes or heated up the closing to form a hump so it will go on. Then some have put something in there for some down force for keeping the pump firmly planted.
These after market pumps no matter who buy them from are an exact science. There seems to be something different for all the applications. We seem to do the best we can with these things.
Dave B.