I got a tip from another sailor during the Belgian nationals three years ago and it hasn't let me down since : use a couple of 1/8" solid braid lines (same as batten ties) to tie the cleat to the mast base and then stretch 'em tight using the downhaul before tightening the screws on the cleat. Leave the lines there, they will now prevent the cleat from sliding upwards. I have since then seen this system used in various races.
Tom