: . . . if you were to puta window in the bottom of a strip built
: kayak, how would you do it?

Here my thoughtsbut I haven't done it. I doubt if many on the forum have so we'll have to put our heads together on this one as this is only a first cut.

I think you have figured out the location, under the deck so the black underside will be reflected.
The window material will have to be flat to avoid distortion. Ideal material would, I think, be a toughened type of glass.
It shouldn't be the type of safety glass that can shatter into hundreds of pieces, it should stay in place even if cracked by a rock.
Non-reflective glass is available and is used for framing pictures.
Plastic would function OK but there is the possibility of it becoming scratched with use, especially when the paddler's feet go by it while getting in or out.
Probably the simplest way to mount it would be to reinforce the opening in the bottom and epoxy and glass tape the window material, preferably glass, in place.
The ideal boat for this would be fairly broad beamed.
Carry a small squeegee . . .

I am wondering if a small double-glazed window unit would do the job, that would have the second layer for safety and solve the scratching and condensation problems. most of the type that open are too thick but a non-opening type might be thinner.