So a year later, my idea has finally came to life. I decided to do it with the upper suggested emergency blankets, 3 of those, sandwiched between 1 layer each side of strong nylon.
The 3 layers are then weaven together at several places. The sheet is then fixed to 4 m long metal rods. The whole thing is 2m by 4m. The way i mounted it, at least for now, seems very sturdy, we havent had a very strong wind for now, so I'm interested when that happens, how will the sheet respong. I weaved the layers with a very strong sewing thread, about 25 sm apart lenghwise, so it should hold up well for a while.
I cant tell if the angle is correct, compared to the sun, but thats what i could do, whiwe using the full size of the sheet.
I definitely can tell that it does reflect quite some light on the shady side of the greenhouse, the base of the greenhouse has warmed up, like it does when directly lit by the sun, just not as warm. The tiles also look quite brighter ,compared to before.
On the inside, the sun facing part is considerably brighter, but again, the shady part that faces the house got much brighter, compared to before.
Anyway, i dont know if this will be much of a benefit, but anyway, it has been done and i will monitor how the plants behave from now on.
The dragon fruits, which face the shady side, are in bud wright now ,should benefit from any additional light.