Foam support

Direct mount pickups (such as the humbuckers in my Ibanez SV5470) are not spring mounted, in lieu of this, foams are used to prop the pickups up. Replacement pickups which would go into these cavities require such foams which aren't sold in any guitar shops, you have to make do with whatever foamy substitutes you can acquire. The material to be considered shouldn't be too soft, otherwise it won't spring the pickups up.