I like to use 6 oz when I find it. Other than that, 4 or so. I do 3 layers and end up sanding about 1 layer off. Use the aluminum sanding bars with aluminum oxide sand paper, 80 grit to rough it in. I like to tack glue with just 4 dots of elmers or tightbond then sand to shape. I found out after severl times that right after you've sanded to shape, brush on a layer of thinned elmers glue (with water) over the foam. Couple of coats is all thats needed. Then fill in with light weight spackling and finish sand. Then a couple more coats of thinned elmers again. Now you can glass the plug right on the plane. It makes it easier to see how much overlap you have. Usually 3/4" is all I need. Too much and its tough to break the plug off. I do all my sanding while glued to the plane. When happy, break it off, pour a little gas in and scrap out the goo, then rinse with water. You'll notice that the inside is nice and smooth. The thinned elmers glue does this for you. If you dont do it, you get a bunch of little spikes that need sanding down on the inside. I let whats left of the goo dry and use a dremil with round and crown wire brushes to finish off the inside. And you're done. Takes me about a lazy weekend to do a cowl. I generally use regular styrofoam. You'll be amazed at how light it is. Very durable. I normally laminate 1/32" ply where mounting screws will go or just add some more glass on the inside after its cleaned out.
Edwin