For what it's worth, here's my kit drying out after a very wet PR match at Borden last May. This kit has evolved over 4 years since I took that first (fateful!) clinic with Barney and continues to develop.
Not shown is the Eberlestock Gunslinger II that it all goes into, battle belt, and my rain gear that was in the dryer. Also not shown was my Eberlestock shooting mat. I've since switched to an Armageddon mat as they are very compact.
I've refined it since then, as the PRS match in New Hampshire had us hiking up and down a mountainside and flying with this kit really makes you want to shed the unnecessary weight. For PR, I'll still take 5 mags, but I only take 3 for PRS.
*Cheap CT rain poncho
*Garbage bag
*Beef Jerky and snacks
*Bug dope and sunscreen
*Rear bag (Gamechanger as well for PRS, although that might replace the small bag for PR)
*Work Gloves
*Ear and Eye pro
*Wrist coach with many different types of markers
*Sharpie
*Rite-in-the-Rain pen and notebook
*Small clipboard (PR only)
*Chamber flag
*Tools - now down to a Fixit stick kit, CLP, and the leatherman
*Lens cleaning cloth and pen
*Sling
*Water
*Monocular
*WeatherFlow meter (added after this match)
*Cellphone for ballistics app (not shown). In this case I'm glad it was weather resistant
*1 x 5 round mag
*4 x 10 round mags
*100 rounds ammo
*permits, letters of invite, etc
*spare batteries
*some empty ziplocs
For PRS I'll take a big bag as well as the aforementioned gamechanger.
Good advice from Chrisward3 though. Start with the basics and add in as you need.