A plate the same shape as the base of the AAV could be fabbed from 1/8" thick plate, aluminum, steel, whatever.  Drill a hole in the center that will just barely fit the cylindrical part of the bulb being used.  Make a gasket the same shape.  Disassemble the original baseplate, break off the peened lip and grind that upper surface flat.  Then, after the piston and cylinder have been cleaned up and reassembled, press this baseplate -- with nothing attached -- into the bottom of the AAV housing upside down, with the recess facing downward.  Finagle a top on your bulb plunger that will hold the bottom of the piston about 20mm above the bottom surface of the bulb flange.  Assemble the whole kit 'n' kaboodle to the coolant manifold is the following order: OEM gasket, homemade plate, homemade gasket, bulb, a thin O-ring that fits into the outer edge of the recess, AAV subassembly.  That O-ring will serve to press the bulb down onto the gasket.  Check to see if you need longer bolts to hold it all together.  This will all locate the AAV 1/8" higher than before, but it's doubtful that will make any significant difference.  This assembly method would make it very easy to replace the bulb if it fails again, but it'll still be a job to get to a stuck piston.

A minor variation on the above would be to mill 1/8" off the bottom of the housing first.  This would put the AAV back where it started, reducing the required length of the plunger.

Another variation would be to find a way to attach the bulb to the plate itself.  That way, the original baseplate could be omitted altogether, which would make it a snap to rework either the bulb or the piston later.  Remember that the baseplate assembly needs to be watertight.

Once the cylinder has been pulled out the top, you might want to machine down the OD of the upper diameter of the cylinder to the same size as the lower diameter, making them both a slip fit.  Then you have to devise another method to hold the cylinder in the housing; might I suggest about three screws threaded in from the side.  As mentioned before, the seal here is not critical; just make sure the spring doesn't push the cylinder out the top.  This would make it really easy to work on the AAV later, even if you leave the baseplate in its original pressed-in configuration; once you slip the cylinder out, it's not difficult to press out the baseplate.