Making a DHW coil?

So I have a 4" threaded fitting welded to one of my propane tanks for installing an indirect water heating coil for DHW. Looking into them, they run a hefty $400 or so. So I am wondering if I might not make one myself as follows: use 50-60 feet of 5/8" soft copper tubing, bend it with say 4' lengths back and forth like a trombone. Solder some bulk head fittings on the ends. Drill two holes in a brass waste clean out plug (they have pipe threads I believe) and then screw this into my steel 4" threaded fitting. Screw the nuts and washers down on the bulk head fittings to finish. Alternatively, I could use copper fittings on the ends of the coil and solder them to the brass clean out plug.

I figure this should cost me maybe $100 or so....

Anyone see and issues with this?

I'm thinking there would be 8 or so 180 bends....I have bent copper tubing before....Worst case I buy 180 bends and solder them in....Still a lot cheaper than a $400 coil....

I only plan to use it for DHW during the heating season. Summers I plan to use one the nyetherm HP water heaters with an 80 gallon tank.

Why trombone? Much easier to make than a coil. I have looked and looked for a 4" dia coil like those you have linked, but seems it is an odd ball size and I have been unsuccessful thus far. Maybe I will get lucky and find one for $100, otherwise, I think the trombone done in 50' of 3/4" copper should work well....I am wondering about the integrity of using bulkhead fittings, but seems that is what some of these tankless coil makers use so they should be ok....

Thanks for the replies....