I started by collecting the parts I would need.

Mechanics

I decided on an aluminum plate from my stock.  This came off a card module that was obsolete and long torn down.  The aluminum was a good size already so I wouldn't need to cut it.  There was printing on one side, but some work with 120grit sand paper cleaned it off, and refreshed the brushed finish.  The aluminum would also help as a heat sink.

I also dug out some small stand offs from stock.  I have so many of this short ones with 4-40 threads that come from panel mounted D-sub connectors.

Instead of nuts, I opted to just tap holes into the PCB.  It makes for quicker construction.  Of course it isn't a very strong material to hold threads, but none of it would take a lot of strain.  In addition to the 4-40, I also used two 6-32 screws to mount the dimmer.

LEDs

For bulk LEDs, you can't beat generic LED strips.  I got 5m of 120 led/m strip for 11usd on aliexpress.  I opted for 24v variety as it means reduced current (doesn't need as thick of wires, and the higher voltage means less of the power is wasted in the series Rs.

I only need 2.5m for this project, and will save the other half in case I like this panel enough to make another one.

The strip was unsoldered into it's 0.5m lengths, and those were cut in half again to nicely fit across the panel.

Dimmer

I wanted something that was simple, and remembering settings.  There are all sorts of programmable ones, which fade, have an IR remote, etc, but I just wanted basic.

I ended up getting one with three knobs, one for each color.  Supports 12-24v, 3a max/channel, which is a good match for the strip.

I opted to add a simple power switch as well.

Power

I already have a 24v 4.5a DC power supply I often have at these events, so it was a logical fit. If I ever need both this flood and the amp I usually use it with at the same time, I'll get another one.

Connections

The supply has a common 2.5mm barrel plug, so I pulled a circuit board mount socket from stock.

I wanted to be able to extend the panel so that the same supply and dimmer could be used for multiple.  I find RJ45 jacks and cables to be cheap and accessible multi pin connections, so went with that here as well.  A simple ethernet cable can be used to connect them.  The 8 pins would allow 4 for power, 3 for the R, G and B returns, and one for ground in case I wanted to power any circuitry on the second panel.

To connect everything together I decided to go with a circuit board.  As it was a one-off and very simple, it was quicker to hand make it rather than having a PCB made or for me to etch it.

The circuit board serves as a mounting, but also tidy connections without having an unreliable mess of wires.