Seat belt attachment points rusted out

[Hawksmoor]

Nasty surprise - pulled on one of the seat belt lower side mount points on my Sprint, and the whole thing pulled away. It is rusted out, and really it only looks as if it was ever tacked on to the door frame metal "spokes".
Both sides are the same - no strength whatsoever in the seat belt mounts, just rusted out.
What is the way to tackle this? I thought of making up a metal plate and tacking that to the frame "wires". Is there a better way?
And what about fitting inertia reel seat belts - is this an easy task?
Many thanks!

[englishmaninwales]

My S3 is fitted with floor strengtheners on both sides, which I believe are from Spyder, possibly the lower part of a roll over structure. It has provision for the lower seat belt mounts.
It consists of an ‘open D’ loop of steel tubing. It runs from an inner chassis attachment outwards behind the seat then forwards through the sill to the front of the cabin then inwards, below the foot pedals, to a forward chassis mounting.
I’ll post up some pictures soon, when I can get to the car.
Malcolm

[englishmaninwales]

Here they are:
Sill and Footbox member http://www.spydercars.co.uk/lotus-elan-2-rollbars/, £188 per side, not cheap but will get you back on the road and keep you properly restrained.

[pharriso]

Not a rare issue, do a search on lattice.

Even Inertia belts use this same mounting point.