I would use no less than 8 lb density myself but that is because I only believe in doing a job once. The 8 lb is nice and hard. You may be able to get away with 4lb if you layer up the glass cap good. Quantities should.be calculated using dimensions inside cavities to be filled and a good online volume calculator. Figure 10-15% extra for waste and learning curve..... with the foam you're damned to get it all out of the container you mixed it in so you lose some there too. Buy lots of mixing containers. Best but most expensive way is to make sure your pour expands over the top, then cut/ belt sand down to finished dimension. As far as bracing, you can use a 2x4 or straight edge braced off hull sides/ other stringers. Word to the wise: do one at a time and be patient!
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