I haven't been able to search for this as I can't even think about how to explain what I want in a short phrase.

Problem
My handlebars are short on space due to interrupter levers and them being quite compact bars.

Possible Solution
A second bar running underneath the handlebars in front of the downtube, lights could then be attached to this.

Second Problem.
To do this I'd need some sort of clamp that would wrap around a round bar and allow another bar to be attached at 90degrees to the first bar.


Two of these attached to the handlebars would then support a secondary accessory bar.


What are these clamps called? Where can I get them? Is there an existing solution I could buy?

Minoura Swing Grip - a quick google search will net you the best price. I paid less than $20 at my LBS.

How I missed those I'll never know. As soon as I searched for it a thousand results popped up with results for sites I had been looking at.

Thanks for your help.

Forget the Minoura Swing Grip. Get this instead. You can mount it like this or this. Or any position you want. And it's pretty darn sturdy.

Your first photo is pretty much how I imagines mounting it. Now I need to decide if a dynamo hub and light is the way to go, 3W doesn't seem that much when I'm a big bloke and could easily carry 2Kg of batteries.

Is there a quick release version of these? I'd love to not waste a bunch of time strapping on lights etc and removing them each ride...

I don't know if this would work for you in the configuration you are describing (or close to it), but I've always liked the looks of the Nitto Lamp Holder. I don't own one...but I'd like to.

They're fairly expensive, though...and do take up some space on the main handlebar.

Shiny.

Leave the bar on (it doesn't weigh much) and put the lights on quick release mounts.

I like your style. Are those light quick releases niterider -specific?

And we have the winner for finding the device in exactly the same way as I would design it.

The only thing I don't like about the Nitto Lamp Holders is the price at close to $50 (not exactly cheap when compared to other similar devices, but one of the best looking in its class).

Take a look at this page on the Peter White Cycles site for a shot or two of the Nitto Lamp Holder with something mounted on it (and other ideas on that same page):

Mounting Lights

Here's another link to where one can be purchased on Milwaukee Bicycle Co./Ben's Cycle:

Nitto Lamp Holder II

Yes. But there are many other lights out there that have similar releases. My current system, for which I don't have pictures, is from Battery Space. They work well too.

Depending on how much extra space you need, and what your desired budget range is, here's another option using two Viewpoint Spacebars side by side. Two of these give you about the same extra working space as one of the Nitto Lamp Holders, but cost less than half the price (but I don't think the Viewpoint bars look nearly as nice as the Nitto products, though). The Viewpoint Spacebars tend to run about $10 each (Performance Bicycle), but I've seen them listed for up to $15 each.

I once needed to mount some extra stuff too and did not like what was commercially available. I took a piece of PVC pipe and cut it long enough to fit between the forward position of my drop bars, just beneath where the brake levers were. I cut slots in each end and ran a hose clamp through each end and secured those to the bars. This made an accessory bar that was about 42 cm wide. I could mount a lot of accessories!! LOL. It worked well and I could place a handlebar bag between the bar and handlebar. Unfortunately I never took any pics of that set up.

The idea is hardly original, maybe with some research you might can find someone else who took pics and posted them.

https://sheldonbrown.org/thorn

Enough room to mount a 'Close Encounters'-like lightbar...

If you have enough room under the bar... put a stem under the stem. It can be a shorty, get one angled down and out of the way. Then saw off an old handlebar
so it's stubby and put that in there. You now have tons of room.