Are you a DIY (Do it yourself) kind of Guy?
Then you can make your own foundry furnace.
But... How good will it be?  How long will it truly take you?  How much will it really cost you?
All that all depends on many things including; is it really worth it for you in the long run to go DIY for bragging rights or penny pinching vs. all collective true costs,  extensive time and  total efforts involved which are all serious questions you need to really consider?

So before you go spending money on some of those DIY plans, or kits, or waste valuable time on what seems to be a free to do plan or a to good to be true plan...
Consider this.

●  If you don't have a complete metal fabricating workshop
●  If you don't have all the metal working tools
●  If you don't have all the refractory supplies and knowledge
●  If you don't have all the materials suppliers found and supply lead times coordinated
●  If you don't have all the needed metals and connecting hardware on hand
●  if you don't have all the workmanship experience to put it all together properly
●  if you don't have all the free time available to work it and make it correctly...

Then Just Buy It, Have it, Use it for Much Less than DIY... and without all the hidden investment hassles above.
Also remember you will be working with fire, fuels, molten metals and extremely high temperatures so quality counts.
You know there IS a reason people buy premade tools, appliances, vehicles, etc.  Same with Foundry Furnaces

In Foundry... Quality Does Matter! So does your Time and Time is Money! Safety Matters Too.
You may find your valuable time better spent using a quality premade professionally designed furnace instead of wasting time trying to make one at greater overall costs, shorter unit lifespan, reduced performance, higher operating costs and sacrifice to other things more important

The truth is even if you have all the tools, equipment, the supplies on hand, and all the knowledge and experience to tie it all together in a non-hit or miss quality way it can still take you a full work week just to make one furnace. That’s right, these are not put together in a day, there are many steps that need to be taken in order with no short cuts allowed including curing times and kiln firing times. Don't get mislead into thinking all this is fast, simple, easy to do thing with a internet DIY plan in hand. Many who have tried DIY are now our happier out of the box premade furnace customers. Still the choice is yours where and how to put your time.  Choose wisely.

Your time maybe best spent on metal casting and all you can do with a quality out of the box ready to use furnace.