Hi Joe. Welcome to the village. I've done a few jigsaw puzzles and didn't think I could do it freehand, but to my surprise, it's very easy. I've heard some folks use Modge Podge, but I have no experience with it. I use Krylon's Triple Thick Glaze. One coat gives a durable finish. I wouldn't try to glue a pattern on top of it though. To practice making puzzles, I glued some old Christmas cards to 1/4" plywood and started cutting. By the time I finished the first card, I felt very comfortable without a pattern. Give it a try. Most puzzle makers don't use patterns. Some use the picture to decide where to cut and turn out some great work.
BTW, I just started turning pens about 8 months ago. It's very addictive.
Good luck with your puzzles. Show us some pictures!