I've had a few Mk1s.. maybe ten or so and none of them have popped out of 5th yet! I had one T Bar which felt it would slip as you could feel the gear stick moving when you accelerated in 5th.. as far as I know it still doesn't slip out of gear though.
I had THE best cure for 5th gear pop out in one of my old T-bars.
At the time my mobile phone fitted perfectly between the gearstick and the gear surround put it in 5th and wedge the phone in