Do these exist or did I imagine it. I’m trying to make one of my Surlys winter ready….

A Crudcatcher you mean?

If you've got mounting bosses on the underside of your down tube then pretty well any piece of plastic sheet cut to size and shape (and sanded down) will do the job. You might need to have washers either side of the plastic to spread the load but apart from that there's not much to consider.

Assume one of those but boltable

Just drill holes in a crud catcher and use plastic washers is my advice if they dont exist( no idea they may they may not )

Probably use a milk carton or some such

I've got a couple of Crudcatchers lying around the garage, with predrilled holes.
Welcome to one for a donation to this here forum.

Thank you but I think I one lurking in the garage, so hopefully that will do the job…