sort of trough to put flowers in

I just got my allotment in November and had lots of plans to grow flowers as well as veggies. However, I've gone a bit mad for the veggies and now don't have any room in my plot plan for flowers.

I was thinking it would be nice to get a sort of shallow trough to sow them in, but habven't had much luck in finding them - any ideas? I thought I'd be able to buy window boxes but can't find any, would you believe!

you could use scaffold boards

... having anything in a container on the allotment is watering the thing.  Can you not plant flowers in between the veg rows?  Down the side of the shed or the compost heap??  Along the fence lines?