Frankly you're much better off buying a pre-made easy entry cart. By the time you purchased all the raw materials, spent all the time researching designs and putting it together, got it balanced, etc., you'll have spent more money than buying a new cart and mostly likely ended up with a heavier, less balanced, possibly unsafe vehicle as well. There are also things that a new driver simply doesn't have the experience to recognize as safety or comfort issues until you've been driving awhile. I know I didn't!
It takes many versions to perfect a design like that and the big companies like Frontier and CTM have had time to do that. Save yourself the time and grief and go purchase a nice easy entry cart!
CTM's are very inexpensive and if you catch them on a sale Frontiers are only $425 plus shipping with a variety of options. Isn't your horse worth it?
I am getting mine specialy made, but at a better price by the Amish and local cart makers.
You can get the measurments and everything off of the websites of carts. I grabbed some pics of my favourite carts and circled what I liked best on them and what I want mine to include.
If you are getting one with a boot, many tack sewing stores will make them but Im sure they sell them some where online??