There are some kits available - for example this one (search “METRA 99-9222”). However it’s plastic and comes with unneeded upper mounting kit for LHD cars. As
@jamesinc mentioned they are pretty easy to fab out of a thin piece of steel or aluminium (I have a metal case from an old DVD player or amplifier for example that has the textured black finish on it so will probably make one for my 79 out of that - never throw anything away LOL!)
https://www.amazon.com.au/Metra-99-9222-VOLVO-MULTI-75-94/dp/B0002BEZ3K