Bruce, you are experiencing what many do when building a router table a-la carte. This is not a bad thing, it just takes a little more research. I notice Rockler states that their "Craftsman Standard" plate also fits the DeWalt 621. IF these are the same holes, then Woodpecker offers their aluminum plate drilled for a 621 as well.
Folks who sell router tables, plates, lifts, etc. deal with these questions all the time. Call Woodpecker and tell them the model of your router and ask what plate fits it. The Rockler plate a $40 is a great deal but this is a decision you will live with for a long time; don't save a few bucks and go with an incorrect plate just because it is on sale.
Learn from others pain and suffering. I have only had aluminum plates; that's just how it worked out, not because I was smart. I have NEVER had a problem with a plate; you will find many posts on problems with acrylics, phenolic and other non-metallic plates on this and other forums.
Just trying to save you form having to buy . . . again. Good luck and enjoy the quest!
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke