So I finally got tired of going to the workshop to fish out my vernier calipers every time I can't decide if something is within the 0.5cm limit for a letter, or 2.5cms for a large letter.
"I know", I muttered, "I can saw up some plywood and bolt the pieces together to make some slots, a bit like the guide they use at the PO!".
And then I thought about how long that would take me... advice like "time is money", and so on. So I had a look here, and on ebay and Amazon.
Plenty in those other places, and at all sorts of prices.
So why doesn't someone enterprising sell any here on ebid? Isn't it something that many sellers need almost as much as parcel tape?