Between the firewall and F2, on the fuselage sides, you need to add a layer of vertical grain 1/8" balsa. Cut oversized, sand to fit, and glue in with CA.

Now make the lite ply piece for the bottom between the firewall and F2. This piece takes a little patience to get it right, but take your time. Since it's at an angle, it's going to be a PITA to get it in. It should be very tight if it fits right. It should rest on top of the 1/4" stringers and butt up nicely against both the firewall and F2. Glue this in with epoxy.

Add pieces of 1/4" triangle stock to the junction of the firewall and the fuse sides, and on the firewall running crossways where the top lite ply piece will sit. Glue these in with epoxy as well.

Now make your top piese of lite ply, this sits right on top of the stringers and should also fit nicely up against the firewall and F2. Remember, the tighter something fits (without bowing or bending the structure) the stronger AND lighter it will be.

While all of this is curing, if you're using a DEPS, install this now. There's really nothing to this except keep it straight. I use the etched teflon sheaths from CST and they glue great and don't have as much friction as the ones that come with the DEPS. (I use the outers in the DEPS as an antenna tube). Just be sure they are glued well where they exit the fuse, and use pieces of 1/4" balsa to glue them between the formers as shown.

The exits are right at F9, a little over an inch from the fuse bottom.

Now is a good time to go ahead and mount yoru antenna tube. I use canopy glue for this.