Learn me adding a lot of accessories to a car

My partner and I are putting the finishing touches on the 1990 Celica that we'll be taking on the Trans Mountain to Tide Water Rally, and part of what I want to do involves adding a whole pile of electrical goodies to the interior and exterior of the car.

I've only got experience using an Add-A-Circuit to add one 12V line from a fusebox, but the Celica's driver footwell fusebox is in a really useless location for doing so, and the Add-A-Circuits cost more than the accessories we're attaching to them ($30/pop in Canada).

Is there a cheap option for this sort of thing? We'll be pulling about 40W if everything is running at once, which I want to do for at least periodic comedic value.

One thing I was considering was going to the junkyard, pulling an underhood fuse box from another car, and then reusing it by wiring the fuse box's power lug to the battery but converting it to ignition-switched using some kind of beefy relay.

I've also seen those fancy switch panels that race cars have, and I've seen generic versions on Aliexpress, but I'm not clear on whether those provide their own relays and power distribution or if I would still have to run a bunch of wires from each switch back to the fusebox/battery anyway.

The ideal end result is that I run one switched-power line to a 'box' and then run all my accessories out of there, like a power strip for a car.

I'd like to avoid vampire taps as well as splicing of the stock wiring, if at all possible. That stuff is doing pretty well on its own as is, and most of it is covered in nasty early-90s Toyota sealant. All of this junk will also get removed from it eventually for either the car's sale or my codriver to use as a daily.

I'm not sure if this is a commonly done thing or if I'm missing out on something totally obvious as I sort of skipped the big-loud-stereo part of my adolescence in favour of stripping my interior.

Tap off the battery with your own fuse and relay if needed circuits? Doesn't sound like rocket surgery to me. If you need more ways to tap into the battery post the audio guys invented a big battery lug with lots of extra little "outs" basically holes with a set screw to hold your wires.

Edit: or run a thick wire to a marine fuse box and power everything off it

This is the sort of thing you're looking for. A self-contained box with relays and fuses. You run one power wire to it and it takes care of the distribution.

First, sketch out a rough idea of what needs to be fused, what needs a relay and how much power each circuit will draw. Then select the right power distribution block. Of course, you'll still need to wire your switches to this box, but at least you won't have a python of wires running back to the battery.