You're going to have to make adamantine wafers and have a dwarf forge them into adamantine armor. All custom reactions make human-sized armors, and this fact is hardcoded; no modding can change it. Since you can't actually get a dwarf to forge things in adventurer mode, you're going to have to do make a fort to do it in.
I'd suggest making 4 custom reactions that work at a smelter. One should give adamantine wafers and fuel, for free. Another should make lead bins, also for free. Both should take no fuel. The other two should take the Weaponsmith and Armorsmith labors to run, and otherwise be free. Then, start a fort in a safe region with an anvil, all the food and booze you can afford, and a dwarf with Weaponsmith and Armorsmith at Proficient level, and just have the dwarf make a couple of lead bins, all the adamantine you need, and then have them train the two smithing skills until the food is almost gone. Then, with his last breaths, have him forge the best-quality adamantine weapons and armor you'd need, stick down a weapon/armor stockpile, and store those things in lead bins. Then, after your fortress crumbles, come by with an adventurer and seek out the bins full of goodies; they're lead, so they'll move relatively little compared to if you left the super-lightweight adamantine equipment free.