I agree, those nuts can be a nuisance. As the designer & manufacturer of the air-filter conversion kits, (as seen on Ebay from time to time) I have dropped more than my share of them. And they always seem to roll way out of reach underneath the car. I think Volvo finally started threading the holes so the nuts are not necessary. Another advantage to my conversion kits is you only have to remove the top cover (one nut) to change the filter element. The rest of it stays bolted in place. Sounds like you have a good idea with the inserts.
If anyone has questions, or wants info / photos of my kits please send me an email: blangdon@gvtc.com