nb Innisfree

We need a photo.

How about a two part connection twixt anti cavitation plate and lid, large dia short section of tube welded to AC plate and same with lid, tube would have a shoulder so that both sections would fit together, large dia would ensure stable and secure position- Unscrew lid bolt, and remove top plate, lift out AC plate. All this assumes that there is enough vertical clearance to insert/remove two short sections. Tubes wouldn't even need welding provided they can be securely positioned before replacing lid and lid has proper sealing round the edge.

Just for the record I designed Innisfree's heating so that the Mikuni heated the rads & calorifier via a two pipe system, it could also be run from the engine coolant via a diverter valve + engine could be preheated by Mikuni (handy in cold weather) Engine could easily run CH using its inbuilt coolant pump. DHW was provided by engine or Mikuni or immersion heater, stove didn't have a back boiler as it would have been too complex to integrate it into the CH circuit and provide a gravity system to protect against stove runaway. All this necessitated the system should be unpressurised, it ran like that with no issues. Quite a simple system in the end which involved a lot of complicated planning & novel ideas!