Remove the crusts from the 1 loaf sandwich bread.
Line a 1.4 liter (1.5 quart) bowl with a piece of plastic wrap that's much larger than the diameter of the bowl.
Mix the 340 grams raspberries and 340 grams blackberries with the ½ cup granulated sugar, 3 tablespoons lemon juice, and a pinch of lemon zest. Toss to coat evenly and let the berries sit while you work on the bread. You can speed the maceration up by vacuum sealing the berries in a bag.
Line the bowl with bread, cutting pieces out to ensure there are no overlaps and no gaps. If the bread isn't very stale, leave it in a windy place for an hour to dry out a bit.
Pour the berries and juice into the bread-lined mold.
Cut some more bread out to make a lid for the berries, ensuring there are no gaps or overlaps.
Pull the wrap over the bread, pressing down on the berries a bit to flatten the surface, and then place a weight on top of the bowl that's slightly smaller than the diameter of the bowl. A straight-sided pot with cans in it works well.
Refrigerate overnight with the weight. Serve with whipped cream or cashew cream.