I'd also go for pacsafe for your backpack, but only for short periods. I started to use pacsafe and I'm very happy. You can add a cobra cable with a padlock passing it around and through the wire mesh, so there is more deterrance. But what does long periods mean in your case?
If you mounted a rack for your board, what if you weld plates with holes in order to put padlocks? Boards are wider in the center, so they want be able to slide/push it forth or back to remove it, nor upwards. I'm not explaining it very well, hope you understand me.
It looks like a surfer and rider community is currently being born. You are at least the 3 around here
(only you on a XR250, pretty impressive).
Safe riding and happy surfing,
PS: About your idea of a lockable cover, there is something similar in international road transport, it's called a "TIR cable" and it's used to ensure that the goods transported in a truck with tarps on the sides (not solid walls) remain untouched during the transi. They put a seal on the cable, so you break it or cut the cable, no other way, but that's not really made designed against theft, but manipulation/swap of goods by the carrier. You could try a similar cable around the tarp/cover over your bike, but you would need a very thick tarp to be any useful or safe. But thick tarps are very bulky and can always be cut. Otherwise, sort of a massive home made pacsafe bike cover, but it'd be even bulkier and heavier. Maybe just use other means to secure the backpack and surfboard but adding a cover to hide them.