[Lozza]

If you haven’t completed this job already, there’s a good constant live from the main feed to the under seat fuse box.

This comes through the bulk head from the battery and terminates with a usual ring terminal at the fuse box. Adding your own ring to that termination would allow you a clean + feed for your requirements.

Visible after removing the sill trim, as in the first photo.

With the seat raised and carpet bucket moulding removed, it becomes clear. Fuse box terminal is just under the white foam block at the bottom of the second photo.

It would save you drilling and provide plenty of space to tuck away your own additional fuse box.