I thought and thought over the weekend on what could be simple enought yet secure enough to deter a thief and here's what I could visualize.
A - Mounting frame for the Spare wheel
B - Bolt
C - Wheel cover
D - Security Cable with loops at both ends
E - Lock
F - Security plate (Frontview)
G - Security plate (Sideview)
You need a security cable(D) like below for this. One end of it can be fixed to one of the bolts(B) in the frame(A) and the spare wheel can be mounted on it. Or you can loop it to the frame itself (prefer a single loop cable). Other end goes through the Spare wheel cover(C). Then you just use a 10mm lock(E) on it.
As I saw, you already have a hole at the bottom of the Cover, right in the center(for water exit) and you cannot pull the cover out parallely. You need to pull back straight slightly then lift the cover from the bottom to an angle till its clear from the tyre at the bottom part and then lift it up, for it to be removed completely.
Reason, the zippers dont go all the way up, there is a dead part of the flap at the top, which obstructs a straight back pullout of the cover, therefore if you can prevent the bottom part of the cover to be pulled out (even with open zippers), you can prevent a theft.
The only thing we need to get right is the length of the cable, it should be just sufficient to come out of this hole once the cover is in place. This will restrict the movement of the bottom part of the cover itself for removal. Top part is already blocked by the dead flap.
The hole in the cover is small, so might need to widen it a bit, depending upon the loop width of the cable. This is the most easiest and basic DIY which I could think of.
Now one can also say, that the hole in the plastic might not hold up, lets say if someone just chips it off or uses force. We can then use a metal L-shaped security plate(F)(G) on the inside of the cover fixed using rivets on four corners tot he bottom. The hole of the security plate should be lightly smaller than the hole on the cover.
The L-Shape has a purpose, if someone holds the lock and tries to pull away from the vehicle, the length of the cable will prevent the movement, if he tries to pull towards the vehicle, the upper part of the security plate will be pushed against the tyre sidewall inside and provide additional strength against the forcefull pulling and breaking.
Now this theory can be used in many ways and even I could think of so many ways to even strengthen it further by using brackets, anchors etc etc but then it would go beyond our basic DIY.
We just want to deter a thief who has no more than a few minutes, a broken cover is of no value to him.