For fork crown and downtube interference issues such as those you may find on 29ers and extra-small size mountain bikes Chris King offers a +5mm baseplate in a stainless steel.
To many, this is commonly called a crown race. Chris King chooses to call it a baseplate as it is technically not a race. It is the foundation of the venerable Chris King Sealed Bearing headset and is designed as a low cost, sacrificial piece, easily replaced when worn or damaged upon removal. Occasionally, it's nice to start with a fresh one - here's where you'll find them.