If it's a bulkhead....suggest you use a wood bulkhead and cover with an aluminum sheet. Then make a pet tern of cardboards, transfer to the new bulkhead and do your thing. A single alum. sheet bulkhead will fit but look like hell and rattle/warp all the time.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."