Hi Robin,
Sealing around corners definitely gets more difficult the tighter the curves get. This video shows how I do it: https://www.diypackraft.com/construction/how-to/v2/step-6-center-seams/
You might consider making the drybag with pointy corners and/or making it slightly oversized, because as long as it’s larger than your backpack it will take on the correct shape regardless of its actual shape outside the pack.