if you have a kayak shop around .

measure the distance of all the backs of the coamings from the
bow & stern, & also the regular measures: length, width bla bla.
then go home & calculate the ratios & I believe you'll be able to come
out with something that will be in good proportions.

or in other words: you're looking for somebody who already did it,
well, the commercial builders already did it.

good luck,

build less paddle more,

Erez