Two possibilities:
Start with a router. Cut a 1/8" rabbet of arbitrary(~1/4") width around the
panel. Grind the rest of the taper with a belt sander, a hand planer or a 7"
grinder. The rabbet will show you the depth. THis will be an ugly process.

Glenn's (justly famous) router scarphing tool might be a good start at a tool
for this. It's limited to 2-1/2" max., so a little touch-up with a block
plane or one of the above tools will be needed.

Rick wrote:

mike worrall wrote:

How do I do this? That is, wat tool(s) would be used for removing the
plywood, leaving a clean cut of consistent depth?

A router.

Rick