@Sallywags Sally are you using it for a Flow hive or a normal lang??
If you already have a flow there are starter strips for the Lang frames - 3 of those flat sides together are the height of the Flow entrance - I taped the 3 three together and made the length 5cm shorter than the gap and it serves well as a reducer.
The one form wood from B&Q you need to cut a notch in the wood one 1 1/2 cm wide and one 5cm wide on different sides, these notches only go 1/2 way down so the bees can just get in or you could make 2 on short by 2cm and one short by 5cm and then use the reducer you choose
Either the B&Q style
Or 3 strips of Flow Starter wood joined together and cut to size.
Does that make sense?