I've got the same boat and the same problem occasionally. There are a lot of trees in my marina and debris is always loosely clogging the cockpit drains. I went to the local hardware store and got some of that aluminium mesh for putting in your gutter downspouts to keep the crud out. Used about half for each drain. Make it a few inches taller than the drain so if some crud blocks the mesh it can still drain. Don't worry when you forget its there and step on it. It's soft. It'll flatten out and you can reshape it when you need to. I pull mine out when sailing. Works well and you don't have to get robbed at a chandlery for it.