With a belt, the first three links (spaces) are left empty, so that the belt can be hooked onto the little protruding tab on the right side of the feed tray (Fig. 23, page 19 of H.Dv. 241), which keeps the belt from falling out by gravity while you try to get the top cover closed. This spacing aligns link #4 (which is filled) next to the feed tray so that it will advance and feed when the bolt is allowed to close on the chamber.
So, if it was me, I would cut off half of the first link of a starter tab to stop it from being able to hold a cartridge, but allow it to hook on the tab without falling out, I would flatten or cut off the next two links of the feed tab so that they won't hold a cartridge either, then I would have a belt piece of 10 functional links to put 10 cartridges in. You're allowed a max of 10, and this way you have the usable belt length you need, but it will only hold 10 cartridges. You could repeatedly use the starter tab on several belt pieces as you shoot.
When I say cut off a link, I mean cut off (grinding wheel, dremel wheel, wire cutters) the curved extended parts which protrude below the connecting springs. Instead of a link being open between 7pm and 5pm, make it so that it is open from 8:30pm to 3:30pm. Everything stays together and functions, it just won't hold cartridges, and therefore those non-functional links cannot be used in counting the overall 10 cartridge length allowable.
They make the rules, take advantage of their rules to the maximum if you can