If everything is in proportion in the sketch, you want to elevate the seat bracket 4.5 inches from the floor. Looks like a column totaling 4.5 inches high, 1.5 inches wide and 1.8 inches deep. If correct, even four of these will be too shaky.
My riser consists of a pair of 2X4X14 inch aluminum tubes bolted to an .090 6061T6 aluminum plate at the top to form a table wider and deeper than the seat itself. It's a Kirkey. With a Kirkey brace bolted to the cross bar, it's very stiff.
I got my materials from a local Metal Supermarket. They have an online store and you can get a variety of stuff ordered cut to size.
https://www.metalsupermarkets.com/