Those pieces are missing on my bike for ages. R&M wasn't selling some replacement, now they do, but you gotta chose between sending kids to uni or paying for R&M parts. So, here's a base for designing your own parts.
 

It's a base, so it has nu function yet apart from sealing the tube. You can iterate on and add clips for catadioptres / reflectors, or any ornament (I'm considering skulls seriously), or lights. The inside can be used to hold a toolbox (like the in-tubes Brompton toolboxes), or a tag, or a rain jacket, or... give me your best ideas.

There is no friction, at all, there's a little bit of void. It can be secured with tape wut you should add friction rings to your model.
 

I'll create a screw-in part for the inside when I have time, so it's a real box and you can put your weed in it. Friend of mine has planned to cram 2 long batteries in it, with a USB plug on the flat outside part, to plug phones and devices while we're doing long trips (we do 75 - 110 km like 6 times a year to reach vacation rentals), that's a good idea.

Please iterate on this model and re-share. I have shared the .step file for iterations to be easy.