We roughed it when I was a kid. Tie some bailing twin around the girth and the inside of your stirrup iron. It's very awkward uncomfortable for a while but ride like that everyday and soon it becomes muscle memory
Stirrup straps work miracles! You can get them at your local tack shop, they are basically a strap of leather that attaches your stirrup to the girth so when your leg flies forward or back you automatically lose your stirrups.
Personally, I find that breeches with grip at the knee don't really help with position at all! Instead, try focusing on sitting deep into the saddle on your approach and really gripping with your calf while forcing your weight into your heel. Also a less exaggerated 2 point over the jump could help that too but it all depends on the height you're jumping. 😁
This probably sounds weird but I almost have to really poke my bum out and not tense with my knees, also feel for the girth with your lower leg as you approach, keep in your mind where the girth is on the horse and where your leg should be will also encourage you :)