Do you want a border? Or must it be borderless?
Only for 16x20 all the time? Or multi-format?
Need a dependable "workhorse", or is occasional use ok?
Absolutly repeatable image size, or is "close enuff" ok?
Fiddle-free?
I've got so many easels I quit counting, some purchased, some home made. Each one suits a particular job; there is no such thing as a "perfect" easel. (Kind of like tripods... you can never have too many).
For borderless prints of any format or size, consider a piece of melamine coated MDF (at your local Home Depot) and a spritz of 3M Spray Mount Artist's Adhesive. It's made for re-positionable graphic layout composition jobs, temporary paste-ups', etc.. Like Jerold says, fix a reference edge along the top and one side using double sided tape. Most borderless easels are not fiddle-free.
For borders, it's hard to beat the Speed-Ez-El, a one trick pony that's a favorite in high production situations. 1/4 inch borders, metal, no-fog yellow color, fast, fiddle-free, & bulletproof. Get one for each paper size. I don't know if they're still being made. Saunders also made a nice border type easel in various sizes.
Have fun.
Reinhold
www.classicBWphoto.com