RE: Screens
First ask the list then check eBay and Samba and you should be able to find
an issue rear bug screen for the rear hatch.  The snaps should already be
there but as you noted no snaps across the bottom.  But when I've used mine
it gave a pretty tight seal at the bottom without the snaps.

I just spent a week parked in my rig and dealt with a moderate amount of
bugs.  What I've found works best for me is rather than trying to keep the
bugs from getting in I make it easy for them to get out.  I open the rear
hatch, slider, and most important, remove the screen from the skylight
opening.  There is a steady stream of bugs flying through but they have an
escape route and don't stay.  Needless to say I'm using bug spray on me,
first in the morning and again in the late afternoon.  I doubt that this
approach will work if in an area that has a big population of flying/biting
things and it doesn't work at night if you close up.  But I still leave the
screen off the skylight at night to allow anything caught inside to escape.

YMMV