If you are confident of your design, your materials and your welding ability then why not - but if unsure about any of the above then don't. I would happily make my own but I don't see the point.
I feel it unlikely you will be able to make any as cheap as you can buy them. Also when you make anything like this consider how easy it is to take down and store. It's pointless going to all the effort of making something like this if it just ends up in your way.
I changed my rear axle with the vehicle resting on a frame under the rear crossmember. It was also bolted to the plate for the tow bar. I was confident that it was up to the job so to put my mind at ease I tried to push the vehicle off - not a chance! Not conventional but I would be far more confident of that than the chassis supported on two axle stands.
The suggestion of some good strong wood is a good idea in my mind - though it will need to form a good wide area throughout the stack to make sure it can't topple.
I have two landrover bottle jacks. Even when I use axle stands these are sitting half an inch below the supported axle just in case. I will sometimes just rely on the jack to change a wheel, but only because I know I won't go under the vehicle.
