I had previously used a bit of tight fitting transparent silicon rubber tube slid over the end of the head of the screw and then up the shaft of the screwdriver in an attempt to hold the head of the screw to the end of the screwdriver.
But it was less than ideal, fiddly as all hell and still prone to dropping the screw into the unreachable bowels of the engine. And being made of alloy, poking about with a magnet was a complete waste of time. Unless you were really lucky, dropped screws were typically gone for ever.
Then I got my hands on this baby - 12" long and picked up for not much $ of Flea Bay. It was suggested by an engineer friend who just happened to have a similar one in his tool box. Oh happy daze!!

Firstly, just 'crack' the screw open slightly with a normal good fitting slot head screw driver. Then insert the split blade screwdriver into the slot in the head of the screw. Then slide the bush down the shaft of the split blade screwdriver which tries to force the two halves of the blade apart, putting outward pressure on outer walls of the slot in the screw. It holds it surprisingly well, enough to be able to completely hold the weight of the screwdriver itself.
In the first shot below, the screwdriver is completely unsupported, and is only being held the friction of the blade being forced outwards against the walls of the slot in the alloy screw.



Screw now securely held by the blade itself

Makes removal and installation a breeze and very fast, esp. those "sphincter clenching inducing moment" inner ones on cyls 2-5 ! No more risk of dropping them at all!
And when reinstalling them, a tiny dab of grease on the top of the washer before putting it on the screw will reduce the risk of it dropping off the threaded part of the alloy screw.
I always just bottom the screw gently with the split blade screwdriver then do the final nipping up with the normal slot head screwdriver, so as to avoid putting any stress on the split blade jobbie, one of my favourite tools in my box.
Enjoy!
Cheers...Tony