Screw support boards to ledges
I then screwed the support boards to the 2x3s ledges that are attached to the panels. 2x4s, 2x6s, or 2x8s would all work just fine for support boards.
Test fit the cushions
The cushions fit great and the sofa is super comfortable.
I finished the frame with a coat of Danish oil.
Cut the 2x10s
The 2x10s should be cut so that they go all the way around the seat cushions. You will need to make 3 panels that are each made from 3 pieces of 2x10.
Sand the 2x10s
Give the cut pieces a light sanding before assembling them. Using the orbital sander, start with 120-grit sandpaper followed by the 220-grit sandpaper. I like to round the edges over a bit with the orbital sander so that the pieces look more distinct and the panels less flat.
Assemble the panels
Lay out three pieces of 2×10s to make each panel. Mark three evenly spaced points along the edges of the boards to serve as a target for the biscuit jointer. Use the biscuit jointer to cut notches in the 2x10s. Insert glue in the notches and then place a glue-covered biscuit in the notches. Use the pipe clamps to clamp the boards together while the glue dries.
Screw on 2x3 ledges
I screwed on pieces of 2x3 to the 2x10 panels. I aligned the 2x3s to the top edge of the first 2x10.
Connect the three panels
I used screws and L-brackets to connect the three panels.
Assemble front support board
I used a piece of 2x10 for the front support board and screwed a piece of 2x3 down the center of it. The 2x3 was warped so I used C-clamps to straighten it out before driving 3 ½” screws through the 2x3 and into the 2x10. I then flipped it over and drove screws through the 2x10 and into the 2x3.