I had a similar LPG system/tank on an A8 I owned a few years back and I considered space-savers, compressors, foam cans, etc. In the end I opted for not bothering with any of them. Drove it like that for 3 years and never an incident.
Ask yourself, how often have you had a tyre that has deflated so quickly while driving that you couldn't make it home or to the nearest garage. I know in my 25+ years of driving, the answer is never. And, even if it did happen, as long as you've got AA or RAC cover, you can always call them out to deal with it.
If you're like me, and you check your tyres daily, it's unlikely you'll ever get caught out. Better still, get a TPMS and you'll have an early warning of any slow punctures, even while driving. I bought
one of these systems a few months ago ... one of the best things I've ever bought. I didn't like the idea of external/dust-cap sensors at first, because I was concerned about throwing the wheel balance off. So I bought some accurate digital scales, weighed them (13-15g) and stuck some additional weights on, directly opposite the sensors and they've been great. Not a single problem or bit of vibration.
Edit: Wrong link. I bought the
TP-8886 DIY Tyre Pressure Monitoring System For Android