Here is a plan for a portable metal cutting bandsaw from the January 1956 Popular Science magazine. This is definitely in the class of advanced hobby machinist. Some would say it's easier now a days to just buy one, but most know that price isn't everything, there is alot of knowledge and enjoyment generated from producing a project like this yourself.
The most difficult is probably the gear reductions, mainly the bevel gear drive box. A easier solution might be to use the bevel drive box from a angle grinder. Some might say "Why not use the whole angle grinder as the power unit?". Speed is not reduced in a angle grinder and the output speed would be to high.
I have always considered one of these, very useful for working with smaller metal shapes. Check out the next post for my solution. A saw like this won't eliminate use of a hacksaw all together, but it comes close.