Three and five point breastplates are very effective for cross country to keep the saddle from sliding too far backwards. HDR and Ovation make good quality ones at a low cost. Magnum Equus does breastplate, girth and bridle sets for $275. Hope that helps!
I use an HDR Five point it's $90ish dollars on Amazon. Another good one is Shires Five Point. I'm a show jumper but I know they're both made for eventing lol