My opinions is it should be none other than Prof. Witchheimer, Adventurer extraordinaire! (And that would be his name) he would be relatively young, ala Brandon Frazier or young Indiana.
He should be in a useful pose, not too much action but not just standing either. He would have all of the equipment neccesary for an adventure, so he would be armed of course, but have a backpack with shovel, maps, antiquities spilling out, a rope at his belt and other equipment besides.
Now, excuse my ignorance, but I'm assuming Prof. Witchheimer isn't an existing character other than Alexs namesake, so if that's the case, it could be a fun project and would give Alex the notarioty and thanks he deserves. The miniature should be sculpted by Copplestone, Mike Owen, Bob Murch, or dare I say it, Todd Harris (my sculptor). My first choice would be Copplestone though as he manages to get good character and good anatomy and I like his stuff, but that's just my opinion.
I'm an accomplished illustrator and would be glad to do some sketches for people to look at and accept or reject.