Yeah we never had an insert either, just the car seat facing backwards until DD could hold her head up properly and then we swung the seat around so I could keep an eye on her in my visor mirror while driving.

I wouldn't think it would be absolutely necessary to use an insert, you should be fine with just a towel or blanket if you're really concerned?

Good luck, I know what you mean about being cash-strapped, DH and I were given a hand-me-down from my aunty but it just looked bodge to us so we ended up giving it to mum and dad to use in their car and stopped off at Autobarn the day we brought her home from hospital with $300 we'd borrowed from the 'rents to buy a new one for us!!