Honestly, we're crap at saving too, so when we bought our fridge interest free on a c/c - it was like forced savings. But you have to stick to your plan, the second you treat it as if its money to spend another day, is when things go downhill.

Honestly, i wouldn't recommend it. If you're going to give it to FIL to look after, why not give FIL $100 every week or fortnight and he can save it for you? It's hard to save, we struggle too, but so worth it if you can make it work.