I built one, using a small section of a SMALL pallet that held a compressor for a walk-in cooler (scrounged from a restaurant installation). Better than Home Depot lumber. 30" wide by 36" tall.
That custom pallet section was made from 2x3, solid wood, about 30" wide. That was the rifle rest section mounted horizontally to the proper height for me and my shooting stance.
Attached to the horizontal rest with 2x4's to the ground, with the 'feet" 2x4's parallel to the target, about 3' long from front to back. My foot position placed within this "track" like a RR track. Screwed together with lag bolts, and right-angle metal braces.
It worked for me when I used it at my buddy's farm.
I plan to build another, using 2x6's instead of 2x4's. I will pad the horizontal rest with a piece of soft carpet remnant.
I'm an old school Cajun, we learned to make things with materials we could acquire, cheap!
Standing rest makes for a proper hold when practicing with big bores.