We also have a 42 foot boat
. We don't have a large double bunk anywhere and it has a traditional V-berth. We have put a t-bag up on the deckhead over the v-berth and use it to store extra linens etc. Might work
for the cushion or even just a net? I like the T-bag because it covers the stuff up so the stuff stays cleaner but has ventilation too. These can usually be found online or at stores that cater to powerboats like Cabelas and other large sporting good (ie hunting and fishing) stores.