Depending on how crude you need to make it, you don't even need a bender. If you use 18ga, you can form it freehand around stuff pretty easy. If you want the rear corners of the tub 'flaired' at the top, then it will get more complicated. If you are ok with the tub being vertical, then a 6-8" PVC drain pipe will work good as a forming buck..
If you want to do it all hand hammered in a more 'formal' shape, you can build a buck out of wood and hammer it out that way.