Is this chamber going to be on an existing straight run? I'd be half tempted just to (carefully) cut the top half of the existing pipe off, use that for a channel. (Obviously pack underneath with concrete to form the base for the chamber.)
As long as the laterals are angled into direction of flow, sit snugly on top of the edge of the new channel, and benching opposite is done correctly to avoid any build up of detritus, I would have thought this would suffice. If the laterals are going to be private (your house only) could you do those in plastic pipework?
As long as the laterals are angled into direction of flow, sit snugly on top of the edge of the new channel, and benching opposite is done correctly to avoid any build up of detritus, I would have thought this would suffice. If the laterals are going to be private (your house only) could you do those in plastic pipework?