There are two main issues about plunderings right now.
The first one is that even if you are granted a support it's still you that have to use a mostly unknown lineup, so, even if your supporter should be able to beat the plunder, this doesn't mean you are able to.
The second one is that if you are one of the top3 of the server you still win always the plunder attempts.
Since convoy/plunder are GROUP activities my idea is that group should matter more in it.
For example i would abolish the support system and simply give only to the convoy runners a boost in stats and initiative linked to a very specific and dedicated stat you can nourish in group (like the snake level up or kept health).
This should encourage weaker people in donating coins to the group and would make plunderings a lot less appealing for top10 people while still keeping the potential maximum reward as 150 coupons per day. (if i'm the strongest in the server i know i win 100% of the times, but if i suddenly begin to win just 33% of the times because my targets gets boosted by their group stats, maybe i begin to do my calculations if is worth to waste hours of my time in doing it or not).