How to support a hollow tabletop in the center?
The trestles appear to be solid wood... and the top is supposed to fit. The tabletop is said to come with predrilled legholes. Regarding the silicon rings (that came with the trestle?), Ikea says, "Plastic bumpers for holding the table top in place are included"; I think that you could drill holes in the trestles and just put the bumpers wherever they need to be- or just use a dowel rod to pin the trestle and tabletop together (since the tabletop has predrilled holes).



I would not drill all the way through the trestle-top because I wouldn't want the dowel pins to fall out through the bottom. I would be tempted to glue it, but then again, it would be easier to take it apart without glue. 

Also, I'm not sure what alum angles are, and I don't see any (good) ways for attaching the table with brackets, but if you happen to have a good bracket for this, then I wouldn't object to it.