How can you make him from existing Perry miniatures .
Well what have we got to start with ?
Apparently Adhemar did fight/ aligned himself in battle and at one stage carried the "holy lance" into battle.
This is not a contemporary illustration but it does show the bishop carrying the "holy lance",another near contemporary illustration below shows the Bishop on his death bed with almost identical headgear.
Other references to use could be Bishop Odo in the bayuex tapestry , this might give us an idea of what a bishop would look like in a war zone.