I had some extra time today, while doing some work on my ATC Bobcat. Your FWC may be diferent.
Here's what mine looks like...
Before you start, it might be good to confirm the location of the where the screws that hold the side, slide, runners should go. Here's a photo on my ATC bare interior walls (without insulation) showing the aluminium frame. Just below the wood, horizontal beam, below the yellow & black wires is the horizontal, aluminium rectangular tube that the screws go into. You might be able to un-do a few screws on the top of your interior panels to peak in there, see what's there on your FWC, and measure where it is from the top.
Here's how mine is attached near the OHC section.
And, towards the rear of the camper.
When you make the bottom of the bed, make sure to have about 3" that will remain on the OHC portion for support when the bed is pulled out. There is a difference in height of the OHC bed bottom and the slide out section, however, with the 4" thick foam mattress supplied by ATC, I never know that it is there, I never feel it!
Oh yeah, don't make the slide out part bigger than the door opening diagonal, otherwise, it might be hard to get it inside.
And, now how does the edge of the slide out NOT bow down ? ? ?
It's the power of the dado recessed cut. The bed bottom is inserted into the dado recessed cut in the cross edge material. Depth of the dado is about 1/4". The bed bottom is 1/2" plywood, the oak edge is 1" x 4". Pretty rigid. The end notch shown in the photos below is where the bed bottom slides on the side sliders.
Left side (slider bed bottom sitting vertical in the middle of the camper).
Right side (slider bed bottom sitting vertical in the middle of the camper).
Oops, I forgot to take photos of the edge all the way pulled out, so here these are (sorry, top was down).
Front edge.
From underneath, back side of edge.
The slide out section is basically supported on all 4 sides or edges. The center may bow down or flex a bit, but, it's not noticable to me when I'm in bed.
I hope that helps.
If you need more photos just let me know what you want to see.