I have several differant type/size strap wrenches. I don't see any of them able to grip the small exposed area of the cap with enough force to break it loose.
I just about live in their tool department and tool catologs, Sears doesn't carry any type spanners, either pin, or ring.
Marine suppliers have an adjustable version called a, "deck plate key" but, as mentioned, being "marine" it isn't going to be cheap. The ones listed in Jamestown Distributor's marine catolog are over $20.00 and may not fit 3/16 holes. And they don't look long enough to give the controled leverage that will be needed to break the cap loose the first time.
The strongest and probably the easest is make one like rocker1 did - just to drill two holes in a peice of steel bar stock and press-in or epoxy two pieces of 3/16 inch steel rod. Then cut the rods to length to make the pins.
I took the even faster way and just reached in the tool box for two Sears Craftsman #1 sized Philips screw drivers. I put the breach in the padded jaws of a bench vise. Then put the ends of the two Philips screw drivers into the cap holes. Held both screw driver handles together in one hand as I pushed in hard, and with the other hand I used a long, heavy screw driver inbetween the two Philips screw driver's shanks as a lever bar.
Paul.