The only things I can think of are:
You might be able to make your own inserts out of Formica/WilsonArt countertop laminates. I would not expect an extended life, however, ad they are designed for indoor use, and UV will likely damage them given enough time.
You might want to contact your "local neighborhood" vinyl sign shop. They may very well have access to the basic material, in which case you can cut to fit... The vinyl may last longer, but, still, I would not expect "indefinite" lifespan. More like 5 or 6 yrs max.
The original inserts were likely applied by the cleat/fixture manufacturer, not Chap, but you might contact Chap for a referral to the mfgr, who -might- have stickers ( or sheet ) available for sale.
Not a lot of help, I know. Good hunting!