[Krait]

Using heatshrink over a long cable run creates a fairly rigid section. Makes it difficult to mould the harness around obstacles and it prevents the cables assuming their own bending radius resulting in a contorted loom.

Heatshrink is only really designed to insulate connections or where the cable insulation has been removed.

As Sideshow says, using split corrugated conduit may be a better solution and way easier to add a cable to later if necessary.

As for fixing, in non visible areas it may be easier to hot glue, or similar, the loom assembly directly to the fibreglass body rather than trying to afix clamps. There are self adhesive plates which have a cable tie threaded through them to hold the loom, they are usually fixed using double sided tape but could be glued in place instead for longevity.

Cheers

Geoff