Until you do get the $$$ for more kegs, you should be fine storing in a cube until you have keg space.
I have stored (conditioned) a Russian Imperial Stout in a cube for 6 months (at room temp) prior to bottling. My last bottle was consumed 5 years later and was better than the first (and not oxidised).
There is the view that the plastic (HDPA) will leach oxygen over time. This may be true, but in my experience 6 months should give you enough time for the consumption of what's in your kegs already (unless you are really slow to drink them). Obviously ensuring the lid and tap seals are good and sealed air tight to avoid contact with oxygen. The storage temp range should be under 30C (the lower the better), but I'd aim for a steady temp of 16C or lower (keep at your lager temp in a fridge perhaps if you have enough room). Avoiding large temp fluctuations is best and keep away from any light. Think like you would for storing bottled beer or wine and you can't go wrong.
