An option, which seems interesting to me, would be to take recycled metal sheets, for example from old appliances, they usually have a very thin thickness and are easily cut with kitchen scissors, you cut the desired size and have a non-porous surface ideal for practicing. and cheap. I think it could be very interesting because that way you practice on something that is more similar to a vehicle, helmet or real object and thus improve pressure and trigger control to avoid spider legs. The other day I did it with the casing of an old gas heater and I took out a few sheets that will come in handy to make a few little drawings.