Seems to me one can make a small 4 sided box out of 1/2 in. starboard and tie a strap over top, perhaps an off the shelf batterry box w/o top will work with some mods.
Don't need such a special formed bracket, the box if snug even with some shims will keep the tank trapped, that;s al it need from sliding around, and a stray over top to hold it down.
Easy to cut and drill up to exact size.
A few holes in side near bottom to drain off any water that enters
Starboard doesn't rust, ever.
I'd use left over 1/2 in. marine ply and resin coat, to cut costs to little and screws hold better in ply.
Some 1/8 in. neoprene as shim.
Just some ideas for DIYers only. Good luck