So I finally got around to getting the instructions for my DIY sofa from my boyfriend and he took a few photos to illustrate some key points. Sorry for the delay folks but I have a terrible memory and have been super busy lately. These are in his words, including the title (which I don't want to take credit for LOL). (Thanks Jeff!!)
First thing is to measure the cushions you are going to use (we used the Ikea's Lillberg sofabed cushions). You want the cushions to fit tightly into the sofa. This will keep the cushions fluffy and in place as you get comfy. The pillows should sit 2-3” below your sides depending on the height of your pillows. You can see the difference in this picture:
FYI – do not screw your wheels/legs to the couch like this. Use proper screws, something I forgot on the trip to Home Depot. And we're going to switch out 2 of the casters on each of the 2 moveable pieces to finials, because despite using lockable casters, they still move around too much.
Lay some 2x6’s lengthwise on top. You could use whatever width of wood you want, heavy is good so the couch won’t move around when you are. You can see in the first photo an end view without the MDF.
The ledge on the back was made by raising a 2x6 so it would make a space for pillows. If you are going to make a pillow ledge remember to take that into account when making the frame (it was an afterthought on this one).