Have you any recommendation for this?
I have seen suggestions of a derailleur friction lever or using two of the three positions of a Sturmey Archer trigger. What do you think?
I have been using two of the three speed trigger controls. I make sure the lefthand one is adjusted tighter than the set up position recommended by SA, the flat on the toggle chain is slightly proud of the end of the axle. If you attach a toggle chain to the axle and pull whilst watching the sun pinions move you will see that once the gear has engaged there is a smaller amount of further movement possible as the spring compresses. It appears that the gears are less likely to slip if the cable adjustment pulls things tightly. I suspect that in use there is a small amount of rocking movement between the sun pinion and the dog ring and that the pinion slowly 'walks' out of engagement, slips a tooth or two and then re-engages due to the action of the spring.