D&D 5E D&D Inventor

I'm trying to create an inventor class (or archetype) for a futuristic D&D game. The sort of thing I have in mind is a Macgyver-type character, who can whip up little creations to help the PCs out of a sticky spot.

What are the best crafting/invention rules people have seen (in any RPG)?

Let's face it, any attempt to codify invention runs the risk of running counter to the creativity behind the act of inventing. While certain material costs can be pinned down, as can labour, your best bet it to find a DM willing to wing it by setting DCs on the fly and a group prepared to work with ad hoc rulings - which includes supporting the DM if a ruling need to be reworked/adjusted after some play/testing.

All that said, and on the assumption you have the above - a Thief can't hurt. Fast hands for some quick traps and tricks!

Sounds like an artificer. Gnomes and Kobold inventor (volos) have a tiny bit of this. I like the type as well but often just deal with the mechanics case-by-case. Interested t see what gets suggested.

Decide how you want the inventions to last and be used. Is this a "The character has invented these contraptions, which allow them to use these effects", "The character can assemble some specific devices at the beginning of the day that will give them these abilities . . . ", or "The character can take a few minutes to cobble together something on the spot from items in their pockets and surroundings.

Is the archetype an Engineer like MacGuyver or an inventor like Doc Brown? Is the stuff they can do something that any suitably skilled person could do using existing skills? Or are they doing unique things with inventions that break new boundaries of science (and possibly the laws of physics as well)?

Rogue, with their Fast Hands, and picking up Expertise in some technical skills would seem to be a solid base for the first. Sorceror or Warlock fits the second, with amazing abilities from devices with certain levels of charge fits the second more.

I have tinker gnomes in my campaign, and one of my characters decided that they wanted to play a gnome that used contraptions instead of magic.

So we used sorcerer wild mage for power level (substituting int for charisma) and all of his inventions replicated spells. The inventions were his way of casting spells and used "focused magical energies" for power. That way we didn't have to worry about power levels or balancing it out, all I had to do was be a little creative with the wild surge so that it fit thematically.

So you might want to take a look at it from the perspective of just reflavoring an existing class. Take an arcane trickster for example and let the character have inventions that would be roughly as powerful and useful as a spell the trickster could cast.

Even if that doesn't work, I'd start with whatever class is closest to your vision and start from that. Add abilities and skills similar to an existing class and remember that looking forward to being able to be better and more powerful as you level up is part of the fun of the game. A first level inventor should be making only simple gadgets, while a high level one should be able to invent truly amazing gadgets.

Ah - using spells to define and constrain what can be done is a brilliant idea! Exactly what I was looking for, so thankyou!