Upper seatbelt anchorage point

[JasonL]

Please can someone who has been through the IVA explain how the minimum height for the upper anchorage point is assessed?

I get the it's a 136x53 rectangle, but is it a block or a plate placed at the centre of the seat?

The sides of the seat in most roadster seats are higher (often much higher) than the centre, so if it was a wide block it would rest on these and would need to be much higher. It seems logical that it's a plate or a thin block, but can anyone confirm?

Explanation: I want a simple roll bar without stays or a seatbelt bar and am planning on welding a 70x70x5mm box section to the back of SB2 and across part of CP3&4 (so it projects up by 50mm) with the bosses going through each wall (outer ones through CP3/4 and the upper wall). Trouble is that it's close to the minimum 450mm.

Thoughts/issues/criticisms of my approach also welcome.

Thanks.