> I don't know why cream humbucker covers aren't widely available- unless it's just for lack of demand.
For Gibson humbuckers, cream plastic covers aren't "historically correct"- but neither are black or white plastic covers.
It's got nothing to do with the infamous DiMarzio trademark- which is for exposed cream bobbins.
> In searching for material for DIY inserts, try color names other than "cream".
The hue you are seeking might be called "tan", "beige", "almond", "ivory", "sand", "camel", "antique white", "cafe au lait", "Swiss chocolate"....
> If you're going to go with open covers, there's no reason to limit the insert material to plastic.
You might find other materials more interesting.
> Formica and Wilsonart both supply free samples to "qualified parties".
You might check them out for some tasteful, or tacky, possibilities.
Formica® Laminate - Cafe
High Pressure Laminate - Retro Renovation® Crème Caramel - Y0406
Found some "creme" polystyrene sheet on eBay.
Different size sheets, all .080" thick, from the same vendor.
Too thick for your application? Here's one example:
STYRENE POLYSTYRENE PLASTIC SHEET .080" THICK 6" X 6" CREME COLOR | eBay
Automotive plastic spray paint?