Update. Southern Filters thinnest F7 (MERV 13 equivalent) filter is 45mm (1.77 inches). They come with card or metal frames. 355mm x 698mm x 45mm (13.9" x 27.5" x 1.8" approx) GBP46 + VAT (sales tax @20%) + shipping. 0.7" seems like a fair increase in depth, but looking at the videos of retrofits there seems to be some free space there. Any opinions?
Got the filter. Southern's service is excellent. And they were very helpful indeed. I went for the 45mm deep F7 (MERV 13) spec filter. The filter arrived as promised. It was safely packaged up and is very neatly made with a simple cardboard frame.
As measured, it fitted the filter shroud behind the frunk bin perfectly widthways and height wise.
Depth. Depth is a bit trickier. I think up to a 30mm deep filter would be fine. 45mm deep is almost the full depth of the filter shroud at the bottom end. This makes it tricky to fasten the frunk bin on easily as the filter housing pushes right up against it. In the end, I weighed up whether to keep this filter in or go for a G4-spec filter at 25mm deep. I kept it in and it seems OK so far. Air flows OK. Unsure if I can tell much difference between filter in and filter out but I suppose I'll see over the next couple of months as the pollen season gets started.
Things I learned.
1. Southern filters can supply panel filters of different filter grades in custom sizes.
2. The whole process of taking the filter housing out, fitting a filter, and replacing the filter housing and frunk bin takes about 40 minutes at a careful pace. I took 1 hour in total as I tried a couple of ways of putting things back together: filter housing, filter housing + frunk bin, then filter housing again.
3. The filter housing is only about 45mm deep at the bottom.
4. There are a few wires and tubes milling around the area near the frunk and it might be possible to squeeze or pinch some with careless reassembly.
5. doing this upgrade is easy with just a 10mm socket driver
6. next time I'll go for a lower-spec filter with more air room
