Initially, I asked Bromton if they would supply me with the plastic part of the bag which slots onto the holder bracket on the bike (I'll call it the holder), but they wouldn't. I wanted to fix this part onto a large plastic bin so I could put the bin on the front of my bike. The only alternative was to buy a bag frame and destroy it for the holder, but they're not cheap and it's a bit wasteful, so I decided to make the holder myself.
Step 1: Making and fitting the aluminium parts for the holder.
I'm not going to detail the making of these parts particularly. If you're familiar with the Brompton carrier, you'll see how the two parts are mirrors of one another and have had pieces cut off from either end to enable fixing to the bin and that a wedge of the aluminium has been removed from the outward flanges of the bracket parts, so the holder will slide down onto the bracket.
The two parts are fitted to the bin with four M4 countersunk set screws. I did this so I could take them off if there was a problem with the design. You could pop rivet the parts on with rivets sunk in small countersinks in the aluminium.
With either screws or rivets, the heads shouldn't get in the way when you slide the bin onto the bike. If you decide on screws, M3 would be better, i think,. With the M4 screws, I had to grind the lower two heads to be flush with the aluminium, as the amount of countersink in the aluminium was leaving very little material.
It's important that when the bin is fitted that the bottom of it clears the front mudguard or front reflector (I removed my reflector to get more room) and that the top of the bin will clear the handlebar. The bin I have is a Curver one and is 410mm high with the lid on. Just try it against the bike with the bracket on the bike first and mark the height of the bracket roughly on the bin.
To fit the holder parts to the bin, I took the bracket off the bike, gaffer taped the holder parts to it and laid it on the bin in position, making sure it was square to the bin and in the middle of it. Then I marked through the holes with a pen onto the bin for drilling.
The inside dimensions of the profile will fit over the holder bracket, but there's a 1mm gap which needs to be filled up with something. I used some plastic banding from a parcel cut to length and stuck on the aluminium with double sided tape. You can see the yellow pack banding in the photographs.
The Brompton carrier frames have a recess where the locking catch fits into the bracket. I have not replicated this on my holder parts as gravity will hold the bin in place adequately and having to reach for a catch to release a bin would be awkward anyway.