Power Supply that will run off of 6or12 volts and output 3.5V with a constant current of 8.1A? It will power a ~22W LED.

http://www.mouser.com/catalog/specsheets/pt54phlatlight.pdf
If my calculations are correct, it is a 22W LED.
I need to power this LED from a 6V or 12V sealed-lead-acid battery. 6V is preferable but I can fit 12V in a dolphin style torch too. :)

I need to create this device, but not pay $60 for it:
http://www.dmxcontrol.de/forum/index.php?page=Attachment&attachmentID=3018&h=a66550fd6e904fd571074e1a60df5af49ee3f303

I claim to have built such a constant current buck-style switching LED driver, although I think I was only running mine at a constant current of around 1 or 2 amperes.  At the heart of this circuit was a big P-channel mosfet being driven by a SG3524, an IC intended for PWM control. 

A 0.1 ohm resistor in series with the load, the LEDS,  is used to sense the current.  Then that signal goes to an op-amp wired as an integrator. The actual signal being integrated is the difference between the current sense signal and a reference, so that when this difference is zero ( or its time averaged value is zero) everything is good.   Basically that's what the feedback is doing in this circuit.  It tries to make the current through that sense resistor be constant and equal to the target (reference) you give it.

I took some pictures of the hand drawn circuit diagram for this.  These notes are good enough for me to sort of remember what I was doing, and maybe you, or someone else reading this, ?might?  find them helpful as well.

You may think this circuit is too complicated, or too simple, depending on your level of expertise.  I think it took me a couple of days to actually build the thing.  You know, time is valuable.  Anyway I think the design I've got here is pretty simple, but it still seems like kind of a toss-up as to whether it would be easier to build or buy such a thing, even if the price is circa 60 USD.

Links to full-res pictures:
https://www.instructables.com/file/FRVNQVWFY3KNS8T/
https://www.instructables.com/file/FEFQS7WFY3KH9QI/
https://www.instructables.com/file/FFTPCVDFY3KNS85/
https://www.instructables.com/file/FDC501YFY5WRLP9/

switchmode-led-driver-1.jpgswitchmode-led-driver-2.jpgswitchmode-led-driver-3.jpgswitchmode-led-driver-4.jpg
worst case, 28 Watts.

Buy a buck controller on Ebay ?

Steve