Sleeper Flower Bed Construction Advice

We are making a flower bed in one of the sunniest spots in the garden and going to use sleepers to raise the level of it so it is more visible from the house. Unfortunately it's also the spot where a lot of builders rubble etc was dumped but I've cleared a strip roughly 17ft X 3ft wide and 1 ft deep. I'm looking at placing sleepers on their edge along the bed edge on top of of the existing lawn on a bed of sharp sand. Given I've dug out the bed already would I be best to drills hole at each end of the 3 sleepers required so I can hammer length of rebar to secure or should I hammer the rebar at the back of the sleeper and secure with fence pins?
Would you line the sleepers with polythene to further protect them or not bother?
The bed backs on to a post and rail fence what backing board would people recommend to use?