Hello everyone. Simple modelling question.

I am trying to model this mat:

As you can see, it’s got a circle shape near the bottom right corner.

How would i go about modelling that?

I have tried this method:

Start with a plane, delete the bottom-right vertice, add a circle while in edit mode, delete the verts i don’t need, merge vertices.

Then i end up with this:

After that, i extrude it so i end up with this:

My question is: how do i cap this so it’s solid?