I've posted to reddit looking for a solution to the "tunnel" question, but so far there has been no response. I'm a bit disappointed, but it could be that it's the weekend and perhaps suitable answers will appear when people return to work on Monday. I suspect that once the tunnel problem is solved, I'll have to join the fore and aft sections with an additional block, use that to cut a new channel through from front to rear (which will automatically create the tunnel) and then split the joined sections to allow for springs to be inserted and to allow for travel left and right.