John Eppley wrote:

Should the "loose" tenons be the same material, or would a more stable
material such as tempered fiberboard be the prudent choice. The fiberboard
is precisely 1/4", does not swell and is very strong. Not to mention, easy
to "store" in the shop.

Try a strength test between the fiberboard and some other
candidates...sugar maple, ash, white oak, etc.

Chris