I knew where to get the cheap plastic bits at about $20 each; I need four. I always expect to pay way too much for an RV part, but $20 for a bit of molded brittle plastic is really pushing it.
My thanks all who replied and especially to wolfe10 for the following suggestion, where 12 of the plastic bits can be obtained for $24 plus shipping. If I live a few years more, I'll probably need the remaining unused 8 of them.
wolfe10 wrote:
$2 each: https://advancedresourcingsales.com/products/cap-step-end
Note: There are two sizes-- need to verify which you have.