Here's good thread on customizing spacers, cush-drives and brake-rotors for wheel swaps.
https://www.garagejournal.com/forum/...d.php?t=187446
If you don't have your own mini-lathe, these guys can supply custom spacers
http://www.flattrackaccessories.com/FWheel.shtml
Depending upon load (weight, power, speed, etc.), making spacers from aluminium can save some weight, but can allow flexing and uneven bearing wear. Titanium is a good metal with densities, modulus and strength right in-between steel and aluminium. Bracing swingarm can help reduce this flex as well.