Or in this photo, Father bike
We finished building the kid seats complete with sunshades.
We used 12mm marine plywood for its waterproof glue (avoid delamination if an edge gets chipped then wetted. I pre-drilled a ton of holes on each junction so as not to split the wood when screwing. I striped the mating edge with gorilla glue then screwed together. So all said and done the bonds should be quite strong.
The order of assembly was seat back to wedge shaped rear facing support. Then I attached the sides drilling from the backsides of the seats, so that the upper portions of the seats were fully assembled. Then I lined them up on the base, clamped, pre-drilled, glued and screwed the uppers down to the base.
Some tips on working with marine plywood, its has more plys for a given thickness, and consequently requires some extra care in working with it. We clamped a backer lock for all holes drilled through the wood such as for the tee nuts, otherwise it will blow out the backside and make a mess of splinters. Also you have to be real careful sanding the edges or routing to avoid splintering it. I even had to saw away from the grain line and use very sharp finishing blades to keep from chipping it like mad.
For the straps I had a local shop put metal grommets in webbing so that I count mount tee-nuts from the back sides for a threaded insert then use a fastener to mount the straps via the metal grommets. Its all VERY secure.
The assembly and prep sanding of the seats took the better part of a day with my wife and I working largely together. The puttying, further sanding, staining, varnishing was done in a little bit of free time here and there over the following week. Exactly one week after starting, we were assembling it on the bike and putting the finishing
touches on it.
The pads are a cordura covers that my wife sewed with a combination of a closed cell foam backing and an open cell foam upper for breathability and shock absorbtion.
