I am not familiar with the various Sony mounts so I am loathe to make a specific suggestion. I am a Canon user and have two VERY cheap sets of 3 extension tubes - yes they are that cheap!
One set has plastic mounts which I only use for light lenses - up to 2 Kilos. The other set has metal mounts (but plastic bodies) which are just fine for heavier lenses - how heavy, I don't know but when I had a 6.1 Kilo EF 400 F2.8 L they were fine and my, current, 4.5 Kilo Canon 800 F5.6 L IS is quite secure on them.
I mostly use extension tubes to reduce the minimum focus distance of long/very long lenses, though when I had a 50mm prime they did let me get up close and personal.