Do you know a good DIY seal/spacer for empty US fin boxes?

Suspect the unused box(es) in multi-fin boards must be creating some drag.

Maybe a slit garden hose might be a cheap option compared to cut-off fins or purpose built spacers. Won't be perfectly flush with bottom, but at least the water wouldn't rush into the slot and create turbulence/drag.

Although suspect there could be some "standard" seal that fits perfectly into a US fin box gap.

just wack a bit of duct tape over it

Pretty sure K4 fins make spacers - if they are half as good as the fins they will be sweet.

AB....

Nylon breadboard chopped up, epoxied to correct thickness and then cut to shape, its only a bit heavier than the same volume of water. So drill holes across to reduce the weight. I did this to fill a gap in a deep tuttle as all my fins are normal tuttle - worked really well.