It's possible to fabricate just about anything with enough time. The question is what is your time worth?I was thinking that I could make 2, maybe 3, jack stands for the price of one of the jackpoint units. Just need to trust your welding.
Keep in mind that the Jackpoint stands are 2 piece and the Rennstand stands are 3 piece. The fitting of the parts together is rather fussy if you want secure support of the vehicle.