I use a large meat hammer... the flat side, not the dimpled side...
Being carefull not to break thru any point, but keeping a uniform thickness through out... about 1/4"-1/3"...
assemble and carefully roll, tucking in the ends...
DO NOT OVER FILL! as that will surely make it a mess...
If I had used a wine bottle, which works well, I would have held the mouth in my hand and used the whole body of the bottle as the hammer...
You want to make sure the pounded out chicken has no "holes" in it (for presentation), but a uniform thickness...
If you do make a "hole", just cut some from the side and patch, from the inside...
you'll never see it in the final presentation...
It did take me some practice to get the pounding right myself when I first tried Cordon Bleu...
But, it was taught to me by a chef friend of mine back in the early '80s... this particular recipe was one of his signature dishes...
Back then I had a 4" diameter wooden wood carving mallet, I got at a hardware store, I used for pounding meat...