If you use foam, please be certain to get foam rated for upholstery use. If you buy cheap urethane foam it will over a 5 to 10 year period turn to either a whitish powder or a gooey mess which you'll never be able to clean up...depending on from which branch of the foam tree it was derived. As I said, be certain it is rated for upholstery use and has a very high ppi (pores per inch) rating.
Buying burlap requires some understanding, patience and study to get the right stuff. I believe 30 oz is the highest available. Here is one source:
https://www.bigduckcanvas.com/burlap/22oz-48-canvas-burlap-natural/ and there are other sellers out there.
Since today it is principally used for hobbies and crafts, most burlap sold is 7, 10 or 12 oz burlap. The difference can be seen in density of the threads. You can hold 7 ounce up and light will shine freely through it. It has the effect of a bunch of 2mm squares in fact. Basically I would have nothing to do with it. By the time you work up to 12 oz burlap, the threads will be next to each other, but 12 ounce still isn't tough enough. I like 22 oz treated burlap. It is strong enough to last longer than anyone reading this. You'll need a strong machine with a good needle if you're going to overlap and sew it (to make a bag, belt, etc), but for this kind of upholstery it should be just fine.
