I see no reason why you can't use a variety of materials for this kind of thing, David. The only reason I wouldn't use the hardboard is if it's not smooth both sides because it may be a good idea to laminate it. Another alternative is to use thicker hardboard and machine the rim from the back to the required thickness, leaving the rest thicker for more strength. Various plastics come to mind, too. I'm actually going to be doing something similar for another 'Creeker shortly and the plan is for a thicker blank with the rim milled down to match the original inserts. I did a similar job for another 'Creeker about a year ago for bandsaw inserts...starboard plastic for that one.
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The most expensive tool is the one you buy "cheaply" and often...