After many years of bulk loading and doing it just about every way possible, I just use the leftover cassettes from a mini lab and tape the film to the stub. I've found it to be the best, simplest and fastest way. If you don't have a lab near you there are people that sell them in bulk on ebay for cheap. I cover the auto ISO grid with tape.
If you are going to deal with 400' movie spools, then invest in a movie film winder. It makes transferring the film to 100' spools incredibly fast. I can have the whole 400' transferred to 100' spools in a few minutes. You really only need one. Get one for 16mm film if you can't find one for 35mm. You can just use a clamp to hold the spool from sliding off.
I have three or four bulk loaders. I use the Watson the most. The Lloyds is ok I guess. The Alden freaks me out a little because the door comes off. I always put a thumb on the door anyway to hold it down, even with the Watson. The Watson wastes a bit of film, but it isn't much in the grand scheme of things. I usually load 40/42 frames as well. Much more efficient.