Here in the states, any furniture/matress manufacturer or factory will have dumpsters full of foam outback, or inside, just ask them. Worst they can do is say no, and it's always free.
Well the lose ness could just be you being hack-attack learning new shit, and your headset could be from all the dust from loose dirt. But I know people who ride foam all the time and there bikes are just fine
Think about it, if its in a hole, it will fill up with water, this on had a drain out the side of the hill but it stil lwasnt big enough. Build it above the ground or your guaranteed to fail
My dads an upholsterer so we are basically making up a huge waterproof sack that we will fill up with foam. It should now need the supports as its designed to stand itself but we dug this hole for dirt so we are making use of it. The pit will be 2.5 meters deep 4 meters wide 5 meters long. Minimum size I know but its alot better than flipping to dirt first time. And big enough to learn tailwhips in. If we have extra material we will the sack bigger so if we ever move it we can have it bigger.
The hole does taper to wards the bottom but that shouldn't be a problem. Hopefully will finish it soon but in the mean time I will stick to freezing cold river jumping to learn flips.