I've been searching for this sort of stuff for months. Of course you hear all the "don't buy Chinese" BS advice HOWEVER on a recent excursion through a wreckers I was looking at relay/fuse boxes in GMs 'cause they use the common square relays and I noticed they're ALL made in China.

So there's a source for cheap and plentiful relays.

Next is the relay box or socket. All the boxes on Amazon, etc look cumbersome and few have built in busses. The alternative is to use individual relay sockets and make your own panel with these. These sockets or the relays can be screwed to a piece of 1/2" cutting board to make a neat panel. Fuse blocks come in a wide variety and so do both negative and positive buss bars.

There are quite a few Youtube videos on how to make a neat panel.

And if you have really deep pockets you should look into MSD's solid state relay strips.