Rick's design involved splices and it sound slike what you are planning is different. More like what Rob makes and Dan Stern shows on his website. That's basically what I made for my car, except made that one permanent, not plug in.
FWIW, in the end I settled on marine in-line fuses directly off the alternator. One for high, one for low. They go to relay plugs that attached to the inner fender. The relays plug in so its easy to swap one out if it fails. For the headlight connectors, IIRC I reused the original ones because the parts store ones seemed junky.
Waytek Wire seems to carry better quality items in general.
I recently bought some terminals and small items from Eastern Beaver Motorsports. I noticed they offer 3 prong headlight connectors and plugs that are pretty compact. In fact one of the items I bought from them were 5/16 wide Packard 58 style terminals. Most of their products are Japanese made of decent quality. Worth checking out. Shipping took about 2 weeks.
Susquehana Motorsports may have Bosch relays. I havent looked in a while.
As mentioned earlier in the thread, Dan Stern and I think Rob both stock and sell better quality connectors and relays. If you just need a few and dont want to hunt, that's probably the easiest route.