Sinar board with min. 4cm extension (for a 155/6,8 Grandagon-N)

[LF_rookie_to_be]

I need to find a Sinar board with at least 4cm extension, in order to mount a 155/6.8 Grandagon-N in a Prontor #1 as far as possible from the (DIY) camera. Does anyone know what is the the longest extension on recent Sinar boards? Could you modify a later-type DB board with wrecked aperture and simply remove the latter and screw in a #1 mounting ring on the front? Black boards look to be sturdier than older, olive-green Norma-era boards, and this is a heavy beast of a lens - and borrowed to me, to make matters even more delicate.

[LF_rookie_to_be]

Are these types of boards sturdy enough for the 155/6.8 Grandagon-N? The board that the #1 shutter attaches to, as well as the 28mm extension collar seem to be made of plastic.