I used Isopon P38 car body filler for a £100 musicmaster I repainted eleven years years ago. I'm sure a nice bit of alder would have been better, but I'd have made a mess of it, having neither the tools, patience, skills or suitable piece of wood.
This was under the pickguard and I sprayed the bass a solid colour, so it was easy to hide and with the pickguard off the repair cannot be seen.
I've never had anyone complain about the bass sounding less 'woody'.