Then find a funeral director who will handle the thing (or _really_ DIY it). Funerals are a disgusting trade, replete with rip-offs and excessive markups. Part of this is the closed-shop attitude by most funeral directors (many of which are now a very small number of chains, operating under the name of old established local firms, whose founders are long gone). There are good funeral directors who are happy to use a coffin they didn't supply (or one they can supply at reasonable cost) but they're the minority.
As just an average furniture maker I can comfortably beat any coffin retail price, and do a better job.