Bush fuschias' roots go down deep, so they won't be happy in such a shallow container. I suppose it depends, though, on how long they're likely to be there.

Re-pot & bring in for winter perhaps? I put begonias & trailing fuchsias in one & nicotiana (18 inches variety) in another

If its only going to be until the new fence is up and you are feeling better I think you can risk it. They don't reach three feet in one season, and if they are cut right back every spring they don't anyway - at least mine don't.

Ideally they need to be in their permanent positions by August at the latest to allow for strong root growth so if you can't do it by then suppose you pot them into large half pots and sink those in your trough - the roots will spread out sideways rather than downwards and you can lift them for the winter, or sink the pots into another border, making sure that the growing point is several inches below the soil level.

I've had two fuschias and a large Camelia in a trough 3ft long by 1ft high and 1 ft wide for the last eight years and it has been amazing. The fuschias came out of a couple of hanging baskets that my daughter didn't want and I had no room for the Camelia in the garden. The blooms on the fuschias are as big as my palm and I cut them back in the Spring.

But Bill' trouygh is only 5" deep - really too shallow for anything but a very mild winter. Delta's Sarah isn't one of the hardiest ones.

Oops, missed that bit!

Lol...

The fuchsias are maxi plugs and I notice root development is very varied. We shall see

Might be as well to overwinter them under cover then for their first winter.