I havent the first idea of where to get sheet aluminium from, or how to bend it properly. I know you can get large sheet metal bending machines.
I hope this all makes sense and you don't need any tools other than a hacksaw, hammer, drill, G or F clamp and some scrap wood.
The aluminium I used was the side of an old saucepan. I cut out a lump that was big enough with a hacksaw, bent it roughly flat and then drew the shape I needed for the bracket on it from a cardboard template wrapped around the radio, cut it to shape and smoothed the edges. The surface of the aluminium was given a rough sanding but as it was being painted it did not have to be that good.
Cut or plane a piece of wood the size of the radio. Lay the aluminium on the "back". Lay some scrap wood on top and clamp the sandwich together with G or F clamp.
Now you can bend the sides in without bowing the back. Most of the bend can be done by hand but to get nice sharp corners finish of with a hammer - not diectly on the aluminium as you will mark it. Use some scrap wood laid on the aluminium and hammer that.
Once you have the sides to shape take the clamp off, place a piece of wood against each side and clamp them on, holding the sides in place. Now you can bend the front edges in. As they are quite short its probably hard to do by hand so, again use a hammer against a piece of wood.
This has probably taken as long to write as it did to do.!!
Finally, buy a RAM mount with a flange top ball (about £15.00) and use the flange as a template to drill a couple of countersunk holes in the bracket. Fit the RAM flange, cover the inside with something to stop scratches (I used some spare wide velcro) and fit to bike.
One tip. Make sure the shape you cut has an opening where the transmit button is located. I forgot and found that when I fitted the radio it pushed the transmit button so had to grind a bit out after bending (hence the slightly wavey left side shown in my photograph.
The Alan radios have a small block sticking out the back where the belt clip fits so when they go in the bracket the block stops them slipping right through. If the radios have a smooth back you will need to bend a lip of aluminium at the base of the bracket to act as a stop.