roger
have made 2 indi seats for caterhams- we both used the medium bag and still had to cut bits off, but neither needed 5" behind to bulkhead. i'm 5'11" 12stone, david's a little larger (apologies if you read this before your ski trip 😬) but both wanted to be below the SLR cage
the instructions are pretty comprehensive but best as a 2 person job. definitely need covering after finishing- i used black fireproof cloth with spray adhesive. it's lasted 2 seasons racing, with driver changes and is very comfortable (for me- can't comment about the co-drivers)
if you want someone to make it for you i know clive at arylium motorsport made one for austin reynolds and it was very professionally finished with a well fitted panel cover, but not cheap i expect
one downside if you use it in all weathers is i don't have a drain hole, so the glue isn't stuck in the concavity now
don't try doing it in a hurry
happy to discuss or show you the seat as i'm just down the M3
jerry