Spacer, what ever is to hand. It needs to be some kind of metal as the starter is earthed through it. So alloy or steel.

You realy will need to add a good litre to the capacity.
More important will be adding baffles for track work.

If your making spacers for starters then have a look at the zetec starter mod. It will make the sump much easier.
Old sierra starters can struggle starting hot zetec's not helped with sitting near a pan of boiling hot oil and the exhaust.

for every thing you want to do there are a dozen different options depending on using your hands or your wallet or something in between.