Thanks, guys! My car needs to get an Engineer's Certificate to get registered, so I figured that once the sound deadening is brushed on from underneath, the repairs will be less obvious. I can't get a straight answer on how strict these guys are, so better to do it right the first time.
The form was made from 40x3mm steel bar, with backing ribs of 13x3mm. The 13x3mm is also used to press the rib in, backed by another piece of 40x3mm. The slot in the form is about 24mm wide.
My house came with a huge old Blacksmith's vice, but any big vice, or shop press should do the job.
The steel for the patches is 1.6mm, all offcuts from a local metal supplier, for a grand total of about $8.00. (The form itself cost about $20)
Click to view attachmentClick to view attachmentClick to view attachmentClick to view attachmentAnd then it's all held together with masking tape, vice grips, whatever you have.
Click to view attachmentAny final tweaking is by BFH.