Good suggestions all.
I'm working on lashing down a board but it would need room beneath it for space for the clamp that holds the camera.
The spare paddle idea would give one camera position. The problem is that the paddle is attached to the spray cover but the cover is loosely fit. I might try attaching velco to the underside of the spray deck and wrapping it around the stern thwart below where the spare paddle is to hold the skirt firmly.
The metal bracket also seems possible. If I understand, the bracket holding the camera would be outside the boat, which could impede my paddle.
I've thought about suction cups, but they could also impede the paddle.
North Water has suggested a square or rectangular opening cut into the spray deck above the stern thwart (yo which I clamp the camera holder). Then, they could supply a square or rectangular shaped two-layer flap just a bit larger than the opening. The underside of the flap would then be glued to the spray deck. The flap layers velcro together on three sides. The fourth sides are sewn together. Opening the flap gives access to the opening to allow clamping the camera holder onto the stern thwart. With the flap closed, the spray deck is sealed. It's an idea but my partner hates the idea of making holes in our brand new expensive spray deck.
I'll post pics when I figure it out.