Go with silicone, use it between the contact surfaces and a thick cord sealing the perimeter. It's going to look a bit ugly but I guess it don't matters in this block
When applied over a clean surface, it's adhesive strenght can be near 20 Kg/cm2. A 5mm thick cord over the perimeter of a 5cm square gives 10 cm2 of contact area, or 200 KG. Using a safety factor of 0.5 it still can hold 100 kg. More than enough for the task.
That's what I used for my block, It's not the best looking but completely stiff. You can see more details about it
here.