I am about to contend with the same thing, I have added slugs to my cruising dacron. As far as I can tell there are two options and I will be trying the easiest and cheapest first. Get a sail track stop bolt from ebay, they cost about 10 bucks. Raise the sail with all slugs above the feeding groove and then insert the bolt and tighten it down. The con to this method is that, with the mainsail struck and flaked, the slugs are going to all be above the feeding groove fairly high on the mast... which might look a little silly and potentially not fit beneath my sail cover... but... cheap, easy, foolproof.
The more substantive option is the track gate which I've looked into. Seems like you have done a DIY version of the gate option. The inside of the gate has to be flush with the inside of the track (think one smooth surface) and the groove width exactly even in order to keep the slugs from binding. This might entail adding some thickness to the inside of the gate to get those tolerances really really close... do a search on the forum, I know I've read a post about it on here somewhere...
Good Luck
