A bit of an update for those that care about such a mundane part...
3 Way Connectors (Paul's link) no longer has them in stock and basically said they're "obsolete" when I emailed them.
I figured I'd give the Proline part from RN a shot and even spoke to 2 different people at RN about the connector and was assured it would work.... It didn't.
I think those connectors are for relay blocks and not the fuse block.
I called RN back to let them know they didn't work and asked if they had part STC4035, which doesn't come up on their site. It's the next wire size version up from STC4034
They did have it!! wasn't too horrible at about $2.50 each
They finally arrived and I was excited to see if they'd work. All went well, they were a bit large for the 16ga wire but I was able to make a secure crimp.
Very happy to finally get my wiring project (heated seats) finished, all came out very tidy and factory looking.
Hearing the "clicking" sound of it snapping into place was so satisfying!