Your Welcome Dave
I just Posted a how to make a mortise jig of the forum.
It's a easy jig to make and it works .
"clamping a second piece of wood " down will work but you will get drifting issues but the jig will fix that.
I clamp all the stock I need to make a book case to the work bench and then cut all of them at one time ,in this way they all line up when you go to put them in place.
Sliding Dovetails can be a pain if they don't line up right on the button as I'm sure you found out by now, they can't be off a 1/16 like the dado.
The jig will keep them true....
and they can't drift off because the jig is in the rail of the clamp.
Just a note:: you can get the alum.stock when you get the clamp and then check it to make sure it fits the slot, the one I found was a 1 1/2" x 1 1/4" in the junk box under the work bench for a equipment bracket ,I also found some in a old alum. lawn chair that worked out just right for the power hand saw.
Have a good one.