The shell was sagging at competition. Stiffening it was not going to happen that weekend, so I carried the load somewhere else.
The carbon monocoque sagged in competition, to the point the car visibly sat wrong.
Rather than attempt to stiffen the shell, carried the load with a separate structure - square steel tube with a 0.125 inch wall - designed and fabricated at the 2022 competition rather than beforehand, and fitted in under 4 hours.
The sag was eliminated and the car sat correctly again, for a weight penalty of about 10 kg, roughly 7% of car and driver together. 3 subsequent generations of the car adopted the approach.
A 7% mass penalty on an efficiency vehicle is a real cost, and it was the right trade only because the alternative that weekend was not running at all. With time, the load belongs in the shell rather than beside it.