They're possible, but would be a right pain in the ar$e to make...
The plates that I put in normally are 30mm (added to the 2x 5mm fascia plates makes 40mm), but if we went for say 25mm overall, I'd end up putting 15mm plates in. As it is, I have to weld the front's blind because I can't physically see inside when the torch is in, so would be even worse if the gap to weld in was 15mm, not to mention getting 2 welds in that space and then somehow dressing them off.
In short, its doable, but would be a right pain, more pain than cutting a couple of slots in the inner wing