My go-to for many years (before I knew about panko) was to use cracker or cereal crumbs. The shape of the crumbs end up being less round, which allows you to get a texture similar to panko.
As they're already crisp, they also work fantastic in baked applications. But you may need to adjust your seasonings based on the crumbs -- crackers already have salt in them, and it's best to stick to an unsweetened cereal like cornflakes or rice chex. Cheezit crumbs make a fanstastic coating for baked chicken tenders.
If you're really set on making panko, however, I suspect that I discovered the secret in a video that dbmag9 posted in a comment in Is panko just pretentious breadcrumbs? -- use a coarse grater. I haven't tested it, and you may need to find the correct moisture level (or freeze it?) to pull it off, but they say "the bread is ground using a proprietary screen" around 3:20 in, and the images look like the "screen" is a huge grater.
The important part here is the size -- if you're not getting large chunks of bread as you grate it (which is going to be trial & error with the staleness of the bread, and might not be easy on a standard grater), you're probably better off just using the food processor and just pulsing it a couple of times, then sifting it so you only have crumbs of the right size. (pass larger bits back though; save the smaller crumbs (aka 'fines') for meatloaf or similar where you're not looking for texture).
(I don't think I saw the Alton Brown video that Jolenealaska had linked to in the comments above, which is now dead ... it's possible that had more specifics ... although a vaguely remember something about panko being made in sheets, not loaves, so maybe that was it, with a different technique).