MJ... 1st you need a lip...measure diameter of your tank...you have single-wall tank, so your lip goes inside (mine's dbl-wall, so lip sits on top)...
You can cut one big ring for the lip or 4 quarter-rings to save material (saves a LOT!!)
Make quarters few inches longer than you'll need so you can trim to fit
Here are the radius arms ... remove router-shoe and screw these back in place ... pivot screw in center measure to router-bit (1/8" plunge) x 2 = finished diameter of cut
short is INSIDE cut of ring, LONG IS outside cut
radius-arm attached to router...screw small end to stock material leaving NO movement except to pivot in a circle...
Cut out the lid ...you'll only need one ;) ... measure from edge of router-shoe to router-bit ... cut template outta masonite larger by that measurement ... sand SMOOTH edges of masonite ...clamp masonite to Starboard, and follow template w/ router ...
Don't try to make it w/just one cut and don't push router too fast ...clear groove on each pass ... takes some time, but it'll make a nice top for your tank ;) 8) ...