This brings up an interesting topic- when does it make sense to make your own tools?
Now some people say always, and others say never.
But some things are easy to make, and some things are easy to make wrong, and hard to make right.
And some stuff you can buy a cheezy chinese version of for less than the raw materials cost here.
Sometimes its a learning experience- and required by lack of dough.
I have heard of old guys who say- I aint got much time left, and I want to spend it making things, rather than fiddling with tools.
And sometimes it is so much cheaper to buy a big old tool used or at auction for pennies on the dollar, rather then spend time and money making one.
A lot depends on usage- I use a hossfeld bender every day, often to the maximum of its capacity. I would not be happy with a homemade hossfeld, unless I spent way more time making it than it was worth- the commercial models, from hossfeld, american bender, and jd2, are all made from a better alloy of steel than A36, precision drilled, usually on a cnc mill, then heat treated, and the dies are made in wide variety of ways- some are sand cast, then machined, some are cnc machined, and some are even handforged- I know of a blacksmith in Wisconsin who forges certain parts for American Bender.
Now you dont need this level of quality if you are gonna use it once a month for 10 minutes- so for lots of people a harbor freight knockoff is just fine.
But I have had mine for about 25 years now, slowly buying and making dies as I need them, and for me interchangeability with hossfeld standards is essential. The original patents have expired, so all three companies make dies that interchange. Hossfeld has the widest selection of dies for structural shapes- T's, angles, square and rectangular tubing, rail caps, etc. JD2 is mostly for auto related, so they are strongest in round tubing dies, while American Bender, started by ex-hossfeld employees, is using modern CNC technology to make a cheaper version than hossfeld, and cherrypicking the most popular dies.
If I was gonna build a quick and dirty bender, I would make it fit hossfeld dies, which means 4" tall and 3/4" hole size, basically. That gives you the maximum flexibility to use storebought dies.
And they have already worked out a couple of thousand different ways to bend things, so it makes sense to use their headstart, rather than try to reinvent the wheel.
I also would buy the hossfeld manual- I think it costs 15 bucks now from hossfeld, and it shows a million ways to bend stuff you never woulda thought of on your own.
Also remember there are several other types of tube bender that might be better for your particular job- a mandrel bender like a muffler shop uses- which is not worth building yourself unless you need a LOT of parts bent. 3 roll type benders, like old buffalo angle rolls- there are plans on the internet to make these yourself as well, crank style. Or a hydraulic ram bender like they use to bend electrical conduit- a 3 point bend, with two rolls and half circle die. Harbor Freight and Northern Hydraulic sell these cheap.