It is not too late to plant it up but there are points to consider. Plants that you put in now will be nicely established by next spring but, if you don't have all the plants to hand, adding extra ones at a later date is likely to disturb the trough. The trough may be nice and light now (empty) but when filled with moist compost and stones will probably be rather heavy for lifting in and out of a frame. It shouldn't need any extra protection anyway. Any planting you do now should be well enough established to get through the winter. Just keep the trough raised up off the floor so that water can easily drain out.

Thanks Bulbaholic.

Using broken up polystyrene packaging can be useful to replace drainage crocks & make planters much lighter. Pretty sure it's a neutral ingredient

Darren that's a good idea. I was going to cover drainage hole with old crocks, could scatter some polysterene then a layer of gravel before filling with compost.