Here's my suggestion.I'll see if I can explain it clearly enough.
You'll need some 3/4" plywood, 1" or 1 1/2" aluminum angle 1/8" thick, stainless 1/4-20 bolts and nyloc nuts, 1/4" closed end aluminum closed end rivets with a grip range of about 3/16"-1/4".
Measure the height of your bench from the hull up to the top, lets say it's 12". Lay out a 12"x12" right angle triangleon the plywood but don't cut it, you'll need to add the deadrise of your boat to the bottom of the triangle and then cut it. Make 2 of these and check the for a good fit where your bench was. Cut 4 pieces of angle to fit the plywood sides that will touch the side and bottom of your hull (2 for each side). Drill 4 bolt holes in each angle with 2 pairs alike. Glue your 2 pieces of ply together and the sandwich them together between the angles with the nuts and bolts. It'll look like a solid shelf bracket with flanges on both sides. Drill and rivet these to the side and bottom of your boat in the center of where your bench was, one on port and starboard sides.
These won't be as stout as the solid bench, but will be a heck of a lot better than nothing.
I'f you have problems understanding my written directions I'll draw you up something later.