Main Fuse in Early Mark 111's
Early mark III's, the first with the drop down fuse tray, used an open, lead fuse, that spanned across two studs and was secured with nuts and washers. On my car this fuse failed because it became loose. The open design is crude and is very difficult to replace. An 80 Amp. AGU fuse and holder as shown is available at most performance sound system stores. With this design, the fuse is protected and can be replaced without tools.
New Fuse and Holder
Left Original - Right New
UPDATE
On newer cars, Superformance uses a fuse similar to this https://www.summitracing.com/parts/bss-ami80. With this new design, the fuse is incased (protected) and will work with the existing wire terminals. While the fuse holder at the top of this page makes for a nice instalation, this newer fuse is much easier to install and very functional.