cut back of cab for subs

i feel you on the doing it right the 1st time...i had the thot of bike inter tubes but for a tractor. lot thicker the making a sealed edge with rtv an a metal rim with pop rivits or bolts. what would u use to make the accordian boot? is that something you would get at the hard ware store or a camper shell place? or a mini truck site?

a campershell place