Some sketches to illustrate how I make one of these tops. I'll try to add some pics later when I'm actually making one.

Before fibre glassing weld all mounting brackets or attachment points to bottom. If you wait to do it after it'll melt and burn the glass.

"Speed Notch" detail:

Pound the end into an oval.

Grind the ends of the oval with angle grinder. You end up with (approximately) 2 straight sides and 2 short, notched ends.

Difficult to see in this pic but the fit is just about as perfect as you can get. The long straight sides make for easier welding too.