The problem with the cap is that the it uses two latches on the cap to fix it to the hood (using inside ridges at the end of of the long hood petals). These latches does not have enough force to keep the cap in place, indeed it is possible to drop the cap merely by shaking the lens/hood/cap (if it's in the "wrong" position). So how to increase the force provided by the latches?
This is relatively easy, the cap latch mechanism is a single plastic construction that can be removed (this relates NOT to the hood with its single release button). This is done by pushing one of the latch buttons all the way in, towards the centre of the cap, and then gently pressing it down into the cap, it should release/pop out of the inside grooves.
Repeat for the opposite latch. The entire latch ring can now be removed. It has the shape of a slightly flattened circle with the latches at opposite ends. At the inside of the push buttons one can see the latch ridges that interlocks with the hood.
To increase the holding force, the latch ridges should be made to be further apart. This can be done by placing an object inside the ring that increases the distance between the latches by approx 3-5mm (the ring will now look more circular).
Now for the scary part, use an adjustable heat gun with a narrow nozzle (10mm diameter) to reshape the plastic. The temperature of the air hitting the ring surface should not exceed 120 C, too hot and the plastic will
melt (forcing you to buy a new cap). Note that there should not be any signs of the plastic ring melting during the procedure and certainly NO smoke!
Carefully fan the centre 90 degrees of the exposed ring at opposite sides for approx 30-45 sec each (NOT the area of the latches, just the this part of the ring). Then leave the ring for a couple of minutes before removing the tensioning object in the middle. The procedure will "realign" the plastic shape, and should maintain its new shape.
You can now test the gripping force by inserting the ring into the hood (best to disconnect the hood from the lens first), if the grip is sufficient the ring can be re-inserted into the cap. If not satisfied the procedure can be repeated with a slightly higher temperature or increased distance between the latches. Don't use excessive force when testing.
The cap should now have a much better grip. I can now lift the lens by the cap, previously the opposite was often not possible!
Note that the above procedure is done at our own risk.