Do you know the make and name of the glider?
There are several things you can do to secure the wings.
1) it is common practice to use electrical tape at the joint between the wing and the fuselage. If this was a high speed aerobat that might not be sufficient. But as a thermal glider it works quite well. That is what I don on my gliders.
2) you can bend the wing rods slightly toward each other. when you put the wing on this creates tension in the rods that helps to secure the wing but I think the tape is likely to be sufficient.
3) You can put bees wax on the wing rods. It is both a lub and a sticky substance that makes it tougher for the wing rods to move. That is also a common action on the tail on a fully flying stab such as yours. I have also heard of people using glue sticks rather than bees wax. I have never found bees wax or glue stick to be needed but they are valid choices. They let you take the wing or h-stab off at the end of the day without difficulty.