Richard.. Before you get the bondo out, assemble the forks, cowl and headlamp shell. The yokes must lie in the same plane, in other words the stanchions must be parallel. If the yokes are not lined up, the cowl will be raised up at one side and the gap around the top of the headlamp will not be even. With everything as it will be finally positioned, now do your perfection panel work, then proceed to the final finish.
Years ago I bought a genuine cowl from C&D Autos when they were still in Birmingham. It was uneven around the top of the shell, so I took it back, got another, exactly the same. It was fine until the wheel was centred with the bars, then the uneven gap appeared. Turned out I had a bent fork leg, lining up the wheel moved the yokes out of plane, hence the gap. The rubber strip was unavailable then so I made a fake one with a piece of P section door draught excluder. Snag was it was only available in white.
The speedo sits in a big moulded rubber grommet, fit to the cowl first, then insert the speedp and work the internal lips of the grommet over the speedo bezel to secure it in place. Soap or washing up liquid helps lubricate the rubber.
Swarfy