Here’s a plan of John’s parallel clamp from his build last night. I’ve increased the contrast a bit from the pencil drawing to make it clearer.
If you’d like to order one, this is the link to Axminster Tools. They also sell then through Amazon, but want £2.95 for delivery – order through Axminster and it’s free for over £30.
There are a number of pre-made clamps available on Amazon (Axminster don’t appear to sell them), although these have metal threads:
If you do make one of these, please take a photo and we can upload it to the site.