I used a huge bolt, nut and some thick washers (thin ones bend). This is the first time Ive installed a 1.5 headset so it was a bit tougher. I recommend 2 large wrenches. Got it all at Ace Hardware store.. You can get a steerer cutter there too! The headsets had little grooves in the lips so they came in quite nicely.
A mechanic/buddy of mine banged a King in with a 2x4 and a hammer and me holding the frame steady once....still works fine after 10 years. I do have the Park press these days to make it easier. The DIY bolt/washer/nut thing I'd do before the 2x4/hammer thing again...
Threaded rod or long bolt, either will work well. However, I suggest you do what you can to find UNF (fine) threaded whatever. For me, this was something available locally at Orchard Supply Hardware, but not at the usual suspected: H.D., Lowe's, TruValue, ACE, etc.
I've tried both, and the difference in ease of installation is remarkable. The wrenches are easier to turn, and the cup seems to press more evenly and more willingly, due to the greater leverage afforded by the higher thread count.
The oversized fender washers I use are large enough to press on the outer edges of Zero Stack headsets. I haven't tried 1.5 like SLX, but it should be no different.
About the only improvement I would make with this method is a selection of diameters of "centering donuts" -- basically discards from a hole saw that sit inside the headset cup or head tube, keeping the bolt/rod centered, but thin enough that they're not exerting any pressure on the cup during the press. You do have to be mindful of the bolt getting off-center, which concentrates pressure on one side of the cup and can cause it to begin pressing crooked. Easier to catch with the UNF threads, but a problem with even a "real" headset press if you don't correct it.
I bought a M8 threaded rod with some nuts 12, 25 and 50mm washers (bout 6 of each) for around £4.00 (can find much cheaper but was in a hurry) and so far I've installed headsets 3 times and it's perfect, the 50mm washers just grab the outside of my FSA needle bearing headset so I get a straight insertion every time. The coarse thread of the M8 rod is not ideal but is fine if you're careful. Now I just need to steal Nate's idea for a headset removal tool as my screwdriver only worked for steel headsets
Just wanted to cast another vote for the long bolt/threaded rod + washers + nuts. I went the extra step and got the King headset and crown race adapters for about $25. Still cheaper than getting an actual press and works great. Actually used it twice tonight...
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