Want to drill & fit Crud Catcher riveted bosses to u/s of down tube – who can do – any ideas?

Use the elastic band thingy they come with?

this may noy be as straight forward as it seems – depending on the tubes used to build your frame, the wall thickness may be too thin due to the butting/tapering of the inner wall.
That said it is a fairly straight forward task for a frame builder to perform Ellis Briggs at Shipley do it for app. £15
ELLIS BRIGGS
whereas Bob Jacksons would charge double that

You have been given advice on this today already,it’s a bad idea but fxxk your frame up if you want to

It’s a good idea if done by a frame builder properly. You will probably want a respray after too.

My advice.

Don’t bother unless you want to drop over £100 to get it done properly.

Buy my Pace RC305 as it’s already got crud catcher bosses on it 😉

Use the force.

I’m failing to see what the problem is with the rubber loop you get with it? Never, ever had a problem with it on any bike, ever. Or are you trying to over-engineer a solution?