I see you got a hat already, but I was going to suggest just picking up a straw hat with a good shape. They’re much less expensive than felt, and you can probably reasonably get away with it in October.
Use a shank bit on your senior horse. Hold the reins in your left (or non-dominant) hand with one finger between the reins. Put your right (or dominant) hand down at your side. My trick for holding the free hand still: grab a couple pieces of chap fringe.
Make sure you know some of the common conventions of the WP show pen: pass on the inside (toward the center), NEVER circle like the hunters often do, and be prepared for reversing at the walk or jog, and being asked to back on the rail.
Have fun!
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