Sometimes it is a royal pain to adjust my easel when I go from large to small canvases. I solved the problem by using some peg board and pegs...while the holes in the peg board do not allow me to completely surround the canvas, two sides is enough to keep a small/medium canvas stable.
I've used many different palettes - gray palette paper, picture frame glass, acrylic, paper plates, etc. This home made solution is my favorite. Approx. 16x19 tempered glass cutting board from Bed, Bath & Beyond; two coats of medium gray spray paint on the "top" rough side - turn it over and use the smooth underside to mix paints - easy to scrape off dried paint. I priced similar (smaller!) palettes on a number of websites. The tempered glass costs a fortune. With coupons from Bed, Bath & Beyond and Michaels I was able to make this palette for under $20...just sayin'