I guess it is unreasonable to try finding it in local bike shop. Here are the dimensions of that part:
width = 13.3mm
height =~ 18mm (it is curved, difficult to measure)
thickness = 1.5mm
hole diameter = 6mm
Some updates. Acrylic glass is not suitable - it is too brittle. Now I’m testing a washer made of 3 sheets of thin (0.5mm) flexible plastic. At least it did not break after initial screwing =)
Hi To you in the U.K., I can help you with a freewheel. I have added the U.K. to my shipping options in my web site because of Strida fans like yourself not being able to find parts. Shipping is 10 business days, a small parcel less than .5 kg is 20.00 cdn.