Barry Perkins‎ From Boston district beekeepers association offers this recipe for Sugar Feeding Blocks

Takes ~5 hrs

5kg white sugar
250 ml cider apple vinegar
1/2 teaspoon citric acid
2 or 3 drops lemongrass oil(optional)
Mix everything together transfer to a baking tray pack it all down, mark lines in it so you can break it when dry, place in the oven or warming cabinets at 40 until dry. DO NOT OVER HEAT.

Also found video to complement.