Unless you pressurize the suit, the weight of the PLSS pulls on this bar protruding from the buckle on the top of the hold down strap. There is no "internal" waist strap on a real suit, and again, unless the suit is pressurized the waist strap does very little to hold the weight of the PLSS, even an empty "prop" one. I'm not sure what you think a whole strapping system is, as there is only the 2 shoulder straps and the waist strap. The front vertical strap is only there to be able to shorten the front of the suit, so when you are in a sitting position, the helmet isn't over your head. It is not intended to support the PLSS in any way. When a suit is pressurized to 3.9 psi, it gets as hard as a basketball, and the entire suit is nearly rigid. Without this "Hold-down" strap in front, the seated astronaut would have the helmet disconnect ring up to his nose, or would not be able to stand up straight. If you look, you will notice, any pressure suit designed to work while sitting, will have this hold-down strap in some form or another. The exception is the new Boeing suit, which uses a zipper system that takes up this extra space in the front while seated.
The design of an item is most often 50/50 - Material / Design (shape) based on the environment or loads the item needs to endure. Change the material or the environment and the design generally no longer works as intended.
In the case of a pressure suit (spacesuit) the entire PLSS support "system" is engineered around the fact that the suit is a semi rigid structure, and, only needs to function in 0 or 1/6 gravity.
Removing the pressurized suit from the equation, means the PLSS now wants sag down, and pulls the suit up in the front and adding 1 gravity, only makes it worse.
The buckle and hoop on the real suit is stainless steel, and an aluminum is marginally strong enough for a prop PLSS, in fact we use stainless to make the hoop part because it gets damaged otherwise.
To be clear, I never criticized Ken's 3d design, just the fact you will need this piece in a sturdier material than 3d printed plastic, if you intend to attach a PLSS.