take a look at RVW's videos for the Roo, the twin of the Shamrock.
i think there are some that shows how to do this.
for an older one like yours, don't know if yours is set up like mine.
on my '07 Roo, i fold the mattress over, wrap the mattress straps around the laid-down C-hoop and snap the straps to the bunk door snaps.
yours may not have the straps/snaps system. don't know how the early ones were set up.
as far as the canvas, just fold it and tuck it in and make sure none of it gets caught in the seals.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC