In a rain-shortened 34-over-a-side tie, Nepal set Namibia a modest 144-run target, thanks to skipper Paras Khadka's 66-ball 44. Opener Anil Mandal chipped in with useful 23.
In reply, Namibia despite 81-run stand between skipper van Der Westhuizen (33) and his deputy Raymond van Schoor (48), were limited to 139/9 in stipulated 34 overs.
Nepal skipper Paras Khadka (44, 2/29) was rightfully adjudged as Man-of-the-Match.
"We always knew it was going to be close. But looking back I think we ought to have scored 20 runs more to put us in a comfortable position. But Namibia fought hard when defending and the turning point for us was when their skipper (Westerhuizen) was dismissed," said Nepal coach Roy Daes.
"We are in the final of the Plate and now we have the chance to defend the title we won two years ago," added the Lankan-born coach.
Nepal 143/8 in 34 overs (Paras Khadka 44, Anil Mandal 23, Raj Shrestha 18; Silver 3 for 20, Strauss 2 for 25, Coetzee 1 for 18) beat Namibia 139/9 in 34 overs (van Schoor 48, van der Westhuzian 33; Rom Shrestha 2 for 23, P Khadka 2 for 29, Vishwakarma 2 for 33).