I've found that the seal at the back hinge is prime suspect for light leaks on most MF Nikons. It's also the easiest to change. The seal/pad for the mirror is also prone to go to goo and is a no-brainer to change. I'd stop there and not muck around with seals around the prism and mirror box.The thin seals running the length of the back aren't critical as light leak sources.

I'd look for thin adhesive-backed foam(black preferably) sold in most craft stores for south of 2 bucks for an 8x10 sheet. Get a straight edge and an Xacto knife or single-edge razor blade for clean, precise cuts.

The real pain is thoroughly removing the sticky old seals. Lots of DIY pointers online.