Need help making backboard (length) for twin bed frame, like a daybed/sofa type back.

[Coyle]

When I decided to go with a twin xl I didn't realize the limitations not having it against a wall would make, so it needs a backboard down the length to give something to lean into, reduce roll off feel. The platform frame is fairly well made, poplar, good design, so would like to keep it. The daybeds I've seen for sale aren't as sturdy in the slat design etc. I have some extra parts from when I purchased the frame that it seems maybe could be repurposed for the backboard with the addition of a little extra lumbar and hardware, but I'm not a carpenter so wanted to post this and maybe get a more experienced viewpoint, as I've watched youtube videos that show fairly simple contruction--perhaps using my extra parts is unnecessary. I'm including pics of my idea of using the extra parts (I'd add the slats in-between and there'd be another board on the other side only had the one to show my idea), pics of my frame , some stock pics of the frame (kd nomad plus) and a couple showing a way to reinforce in the center sections. I really only need the board to go down the length, the sides I'd just as soon leave open if possible but understand might need them for support. I'm thinking maybe some L brackets, drill straight thru with bolt, washers and nuts, and perhaps pocket screws, which I'm totally unfamiliar with. In the past my method has been 90 degree support brackets, bolt, washer. Have to get this project done though so my child can sleep in it, as it is I'm concerned could roll off as is used to having a bed against wall. Thanks, Coyle.