Adding planks underneath rather than on top feels slightly strange. I clamped on a couple of brackets to support the plank while I clamped it in place.
In the picture below, you can see the line of epoxy resin filling the groove in the plank, all ready to be fitted in place.
The floor provided the anchor points for the clamps. I no longer needed my set of straps.
The curve of the plank needed more clamping closer to the deck shelf at the bottom.
And finally, the gap has been filled. I planed the bottom plank before fitting to match the edge of the deck shelf.
Now time to start on the other side!