The wedge should be angled to let you put feet ahead of your body, knees fairly straight, not bent like a chair seat. You need to be able to reach the floor and have SOLID footing, to press into your feet.
Adding height to the wedge can help if you can’t see ahead of a tall horse, for looking down the path, road, where you are going. But every wedge doesn’t NEED to be tall, just higher in the back than in the front of where you sit. The angle is what helps get feet and legs out ahead of you, not the height.
You really don’t get to use the backrest when you add the wedge! Sorry, but the Driver will learn to use the upper body, leaning forward and back a bit, which can pull the reins or release them, beyond what moving your hand gives the equine with movement. Does save some rein adjusting that way, on straight-a-ways.
I would go SIMPLE to start my wedge building. Start with an angled square of wood boards. Again, lower in front than back, start short, to see how you like it. So maybe a 2-3 inch front, with a 5-6 inch rear. Put in some cross pieces on the top of square to hold the flat cushion, put it on the vehicle, go for a drive. A wedge should be FIRM, almost hard, no give. Sorry again, not the most comfortable seat! Cushion should not be slippery, so the Rubbermaid shelf liner with little square holes, is a good cushion topping, to hold you in place on cloth or plastic seats.
Going for the drive, do you feel SECURE in that seat? Can you see? Feet are solid, tight to the floor so you can push on them, not move?
If the wedge frame doesn’t do what you want, make another with height changes you think would be helpful, try stuff again.
Sometimes with a wedge addition, shorter legged folks will also add a wood box for their feet to get the firm footing they need. Paint the foot box black, put carpet or rubber flooring on for grip, fasten box down Securely, box almost turns invisible to notice but you have the good footing you need.
With our wedges, we have used both paint on the wood frame with cushion above, and covered frame with seat fabric, put a solid seat on the frame and covered it with matching fabric. You just want that wedge seating to be firm, not let you sink into cushion, rock around on wedge as upholstery gives under you when you move. For more Traditional type vehicles, the wedge won’t have sides, according to our antiques what came with old wedge seats. Just that square frame, upholstered to match the other seat fabrics. This is on 3 elderly carriages, with original upholstery, wedge seats.
Wedge seats with sides, higher edges, are a modern thing, evidently to hold the Competitive Driver in place on rough ground. Not a requirement, some times not even desirable. Because if you bounce UP, you MAY come down on that higher side or edge and it HURTS.
Making these seats, a wedge, you have a big advantage of being able to change things that are not working for you, not having a lot invested, should an idea not work out precisely as you planned. You might also consider putting the wedge in the seat center, with small cushions as spacers, for driving alone in the vehicle. Does give a different view than sitting on one side of the seat! Heck it is just two more little cushions!! Ha Ha
Something to consider is if or how, the cushion/s and wedge are anchored on the seat frame. Tight fit helps hold things in, but straps are an acceptable method of holding things down, especially if using rough ground to drive on or compete over. Old reins are nice brown, long, you can put a buckle in the rear to open the straps for seat removal during travel or for storage. We use footman’s loops for the strap anchor points or to hold straps straight if you run strap completely around the seat frame.
Husband the driver, uses wedge seats on everything he drives. Feels it improves his body position, secure feeling in his seated location. Depending on the vehicle, they are in different heights for the wedges, nothing firm in what works best for ALL vehicles.
We learned what worked by experimenting with OUR vehicle and horses. Our end result would not be what works on another vehicle or their horses. The wood frame wedge to learn angles that work for you, is fast, inexpensive, easy to change around as needed. So go for it, find what you like before doing the upholstery stuff.
I really think you will like having a wedge seat, after a bit of use to get familiar with using it.
