Andrew, the 50mm bottom supports the weight - which is of course, quite substantial. To be fair, I also had lots of it lying around at the time. I simply shaped the sides of it & screwed the plywood sides on. Also, where I was pouring, the ground was quite sandy & I was worried about movement. The last thing you want is some undesigned rocker in your keel. Better to design your mould too strong & not have a problem, than to have it bow/move & have to start again. If you have to pour on a concrete floor, the thick bottom allows you to block it up on bricks, whilst still keeping it flat. If you do pour on a concrete floor, 1/2 a cubic meter of sand should be used to make a levy, in case the mould lets go....

You have to make sure the bath isn't too thin - but remember, they do support a fair old weight in water when full. But, an old, rusty one may be suspect. So what if the enamel comes off in the heat? You worried about it contaminating the lead? In all probability it (along with practically all other contaminents, such as silicone, aluminium, iron/steel, etc) it will float to the top of the bath, where it can be skimmed off before the pour.

Yes, of course you have to allow for shrinkage - this is a given for all metal casting, regardless of the metal type or application. I can't recall the contraction rate for lead off hand (0.8% for cast iron, 1.25% for aluminium... never forget some things....) - we certainly didn't have contraction rules for lead handy in our pattern shop - but whatever the rate is, for a long casting like a keel, I'd add an extra 1/4 of the rate to the longitudinal dimensions - the contraction will be a bit over the 'standard' in the length... So, if the contraction rate of lead is 2%, I'd make all my 'long' dimensions 2.5% greater...... You can always cut/plane/file it shorter, but hard to make it longer... (although not impossible!).