I have been wanting to get one of these for my dad but held back because I felt like surely there must be a way to do one of these things and have it feel more personalized. Thanks to the internet, I found a pattern. I have my 3 yards of fleece but can't confirm how well it works yet but it looks like it should work pretty well.
Here's the link:
Search,"homemade snuggie", if the link doesn't work. You will find a link to the pdf pattern by Welmoed Sisson.
The pattern works! Just finished making it with the help of my mum. Three yards is plenty of fleece. The trickiest part are the arm holes. We made a template on paper and pinned it on the fabric and cut out. Pinning/stitching the arms to the holes was finicky but not impossible. The arms came out really long so we made a big hem and still had enough to roll up as a cuff.
Thanks to my husband for modeling. =o)