I spent a good few hours doing front speakers (and amp etc) last summer on my Mk4 FR and would suggest the following to keep life very simple:
1. speaker lead adapters - to plug aftermarket speakers direct into the existing wiring adapters - search ebay for "PC2-807
Seat Ibiza Cordoba Speaker Adaptor Lead" - got mine from liquidice, about £3 for a pair - they say for
ibiza 1999-2002 but were correct fit for the mk4 2004-
2. MDF Speaker rings/spacers - depends what depth you need to achieve for your speakers. I fitted Hertz DSK165.3 which have a 64mm mount depth and I calculated I needed 22mm spacers. MDF ones I got were from radioworld-online on ebay - search for "RW-SR-20" - they have other sizes on same auction, about £7 for the pair. They were a close fit but ok - about 2 mm gap to window, doorcard fitted back on fine.
Unless you really have the time/skill to build your own MDF rings or need a very unique depth it's a lot easier to just buy them - just measure carefully to get the right depth
