This is one type. It should help you narrow your search. https://www.amazon.com/Plastic-Spacer-Thickness-CONCAVE-Washer/dp/B0816NDPVF413
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This is one type. It should help you narrow your search. https://www.amazon.com/Plastic-Spacer-Thickness-CONCAVE-Washer/dp/B0816NDPVF413
Could use Maker Pipe T connectors there. If you are trying to keep it slimmer without a connector, why not just file the pipe end with a rounded file into a curved shape, or use a Dremel grinding bit. Or if you have access to a 3d printer, curved endcaps would be an easy model.