Yeah, not exactly 'cheap'. I just watched the auction sites and bought the pieces separately as the prices got 'acceptable'.
The depth of the inserts is just shy of 10cm so I'd go with that as the starting point for your home made solution.
One thing I noticed when measuring is that the inserts are contoured slightly to account for bits like the anchors and whatnot.
If it helps, my right insert got damaged and I'm looking to replace it. You can have it and either try to repair or use it as a template if you like?
I'll be at beach dubbin on Sunday (Portsmouth) with a bunch of bits from my last GTI (RIP) for sale (I'm not in the trade, just doing a clear out).