You can buy the high density foam that is used as the core of those blocks from rubber/foam suppliers just use one of those hot wire cutters to cut the hull shape, mine are perfect. Make sure that the bungy is only just tight enough to hold it in place one of the F16 guys nearly lost an eye a few years ago when he was attaching it. Most of them have a bungy that goes completely over to the other side which makes you lean over with your face next to the bungy, better to modify it so it hooks together at the top