I've had a few different rear lights on a few different ebike conversions, this is the best cheapo one I've found so far;
Linkbest M9
Nice and bright without being a glare. Good quality, very visible from the sides too.
Linkbest M9
6.64£ 51% OFF|Ebike Helmet Taillight Lantern | Bicycle Taillight Led Dynamo - Bike Light Waterproof - Aliexpress
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Nice and bright without being a glare. Good quality, very visible from the sides too.