Help with fat sacs

I'm looking to weigh my boat down a bit in an effort to increase the wake. I've got a 96 Tige without a tower, so my boards are all over the place. I was looking at the various fat-sacs that are available and thought the fat seat might be cool. I'm a bit concerned about how tough the sacs are though - will a wakeboard fin break it open easily? If you go with the regular fat-sacs, could you use them as seats or will they break if you sit on them? Also, how long does it really take to fill them up (say a 500 gallon)?

Thanks.