This is my first comment on a server, so please forgive me if I break any rules/etiquette.

This being said, I have been there and thought of using eva foam and high density foam and then gym tiles and such. I did the math on raw material cost vs a target I could buy, like this one here. In all the cases I got the same price or a smaller target for a touch less (10 euros or so), couting in just the foam price: not the personal time needed and the other inevitable extra material needed to make a good target. So i did not even bother to try.

Anyway I still wanted to make a big target (as I already made some small targets in the past, but suitable in my opinion for more than 25 meters). The first targets i made were a layering of random packaging foam and old school notebooks inside a cardboard box, and they worked quite well stopping arrows from a 45 pounds bow @ 15 meters, but they deteriorated quite quickly.

I thought of using plastic material, but I do not like the idea of small pieces remaining on the ground, but mainly those materials are very good if you pack them right: they need to be squished hard and tight inside the receptacle, and it is hard to be even by hand.

So my last idea (and this experimental) was to go to the local supermarket and ask them for cardboard they throw away: cut it into small pieces and mix it with PVA glue and water. Normally this glue would get too hard to shoot against, but you can avoid that by adding a ceratin amount of veg glycerin. I still have to try this one and figure out the the tright amounts. I'll keep you posted when i have the time to do that :D