Ive now received the tape measures, ordered from Amazon (these here - 7.5m-25mm-Measure)
The design ive decided to go with is one that can be found on the AMSAT website. Heres a link to it in a PDF format W6NBC 2m/70cm Beam.pdf Its not strictly a dual band beam - its two beams - a 3-el 2m and a 5-el 70cm, that clamp together in a crossed yagi pattern for satellite work. The 2m beam on its own can be used for ARDF.
The tape measures are 25mm wide, which means that a full size 145MHz director element is self supporting and does not collapse even when upside down, but still easily deforms if caught on foliage, or for stowing away. The tape itself cuts smoothly with a pair of aviation type tin shears.
Ive rounded off the ends, which is essential, as the corners when cut are razor sharp! Do this, AND cover them with insulating tape! That way, you wont hurt anyone, or yourself. Make sure you pick up all the offcuts - they are like needles! You will not be popular with the family if someone gets one in their foot!
The picture shows the 2m antenna elements cut and ready to assemble to the boom (which ive yet to form). A narrower tape will be used for the 70cm beam elements, which are so much shorter and so dont need to be as substantial.
The measurements, being American, are all in decimal inches. I found it easiest to measure the length to the next whole inch, then once cut, use a digital caliper to measure and trim the tenths.