This may seem weird, but last Friday I had my first proper multi-mode commute. I have been on the train with bikes before, but never rush hour.

Friday night we had the Fleche-Opperman trial reunion dinner in town, I rode there after work and took the train home at 10:35pm. I thought the train would be relatively empty. Big mistake. Even standing room was in short supply. I was VERY glad for my Mini which sat folded in front of my legs.

I am still looking for a practical way to clip the folded bits together. I fiddled with a magnet before without much success.

I use a bungie cord. Works great and only adds 3-4 seconds to the fold time.

If you have a rear rack, you could permanently tie one end of a bungie cord to it, and park the hook end to the rack until you need it to wrap around a stay or whatever.

I just remembered that one might not have a rear rack on a DT mini.

How about cutting and attaching one end of the bungie to or under a waterbottle cage.
You could wrap the bungie around the cage and park the hook end on it when not in use.

Good idea; I'll check it out.

I'm also thinking about some sort of plastic clip-in retainer scheme such as used on the Brompton.

I have experimented with using HDPE in a similar manner to that described for clipping in aluminium tubes in folding kayaks like this https://www.yostwerks.com/HDPE0.html with some success. All you need is a small piece, maybe 5 cm by 7 cm and drill it at the correct angles to match your fork and rear seat stay. You drill a hole that is the diameter of the tube but offset it so that the hole is only partially on the piece of HDPE which allows it to snap on and off the tube. Works pretty well on a single tube, but for multiple tubes you need a pivot with a hole on each piece or measure the angles that the fork and seatstay are at and then drill the HDPE accordingly.

For those who aren't familiar with HDPE it is the plastic often used for cutting boards.

I just use Velcro one-wrap on my folders because it is bombproof and cheap.

Velcro strap.