I'll let K.C. respond on the proper use and care of shallow plastic bins.
About "thwarts", that's what these reinforcements are called, if your boat doesn't have them, it is because your CB trunk had some "buttresses" added that are absent in later boats (you might be able to tell whether they are original to your boat or added by a previous owner, but only if, in the latter case, the transition to the CB trunk or hull is of a different quality than similar transitions in your boat).
I find these very convenient for crew to sit on when we are fine tuning boat balance in lighter, or very moderate winds. By sitting on either thwart, the crew can pick the best spot so that their weight lets me lean back to see the tell-tales without heeling the boat to windward. (If they sit on the windward side, in very light winds, that gets a bit in the way of the jib sheet; some of my various crew hate that, others manage fine.
If you wanted to further stiffen your boat, you could add your own. They are attached to the CB with a metal L bracket, and I don't see why you couldn't use the same for connecting them to the seat; you may need to divide the bracket in two and/or bend it a bit beyond 90 degrees because the seat sides aren't plumb, if I recall.
You will have to cut an opening into the front of the seats for an inspection port to be able to position a backing plate for bolting the L bracket, unless you glue it to the seat with epoxy (in which case it needs to have as much area as you can manage). Bolted and glued would be my recommendation as the thwarts will pull on the bracket as the boat flexes in the water.
You may want to cut the inspection ports anyway so you can check for soggy foam under the seat and replace it with pool noodles. For that operation, you need the largest deck plate you can fit. I believe the outer diameter of those is about 1.5" wider than the rated diameter (opening).
If you decide to add thwarts, you need to measure them with the boat in the water; left and right will most likely not be the same length - you'd be amazed how loose the "tolerances" were in building these boats
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. The hull will open up when supported on a trailer and close up a bit on the water, so you need to measure the latter.