Options for carrying a smart briefcase on a bike (pannier or otherwise)

I do not write from experience, but I have seen pictures of a Danish rear rack with a hook for suspending a briefcase. Seems like it might work well for shortish distances, although I wonder how a large a bag can be used before striking it with the heel becomes a problem.

Here is an example commerically available:

https://velorbis.com/product/all-products/rear-carrier-for-classic-gents-bicycles/

Such a rack might not fit on a folding bike.

Tie the carry handles of the laptop bag to the saddle rails (the small bars under the saddle by which it attaches to the seat post) with an old inner tube, bungee cord, carabiner etc. The bag itself rests on the rear rack.

This would still allow the handles to twist and the bag to move to hang beside the saddle.

One solution is to make small holes in the bottom corners of the laptop bag and tie the bottom corners to the sides of the rack further back than the resting place of the bag. These ties would have to be fairly non-stretchy. This would prevent too much side-to-side movement, keeping the bag on the rack.

Another solution is to use the shoulder strap or its attachment hooks to prevent side-to-side movement. In this case, when viewed from the rear, the left hook of the bag should be tied to the right side of the rack and vice versa (in an X shape). The left-top-to-right-bottom tie prevents the bag from moving left on the rack when viewed from the rear. Symmetrically for the right-top-to-left-bottom.

A bicycle trunk will probably fit your bag:

I have used one in combination with panniers:

It was rugged and stable enough to handle 15kg of luggage on mountain dirt roads with rocks, mud, sand and sharp turns.

You want a way to carry your briefcase on your bicycle.

I do not think there is any way to attach your briefcase directly to a bike rack. And in my experience using bungee cords to strap a backpack to the top of a bike rack, the backpack kept on slipping loose. It was generally a pain in the butt.

However, Banjo Brothers make a folding grocery pannier which allows you to carry bags of groceries on your bike. This would probably be your best bet. Banjo Bros make very high quality gear, and this pannier can fold when you are not using it. It's also only $40 (relatively cheap as far as panniers go).

The only problem is that it is not long enough to fit your briefcase. But based on the dimensions (from their website) on the picture below, if you put your briefcase on its side, it should fit snugly, with only ~16cm sticking out the top. And because the pannier is intended to carry (heavy) bags of groceries, you don't have to worry about your briefcase sagging.

Rather than trying to balance the bag on top of a rear carrier, you'd want to attach it to the side. However, I'd recommend against this. You have a nice bag and it will get scuffed up from rubbing against the carrier and also will get dirty from general road muck being thrown at it, especially after rain.

Instead, I recommend leaving your nice bag in the office and using either standard panniers or a rucksack to take your stuff into work. Unless you put your bag inside some other bag, it will, frankly, be wrecked by cycling with it.

Your bag appears to have a shoulder strap - its perfectly possible to ride with the bag on your back and the strap on one shoulder and under the other arm.

However there are downsides:

1. The bag will encourage sweat in that one area where it presses.
2. The bag may suddenly shift downwards, altering your balance.
3. The shoulder with the strap on it can get achy after a fairly short time.
4. Bag will be exposed to weather, and road spray if you lack a rear mudguard. It will need more-frequent cleaning, and periodic weatherproofing to keep rain out.