I got a pic 
Anyway, I think you should have a base named after u better then a spar arena, ur gang ain't good at spar but better at basing |
The only time I remember doing that was a long time ago, and we were bored and sparred our subs, not boosted.
But I believe a spar arena would not be a good reward for a basing gang, as johnaudi said, maybe something that matches the achievement better.