Remember, it's not the size that counts but how you use it
My first dio - "The Tale of Zartan and the Grape Soda Machine" had a set that was just folded bits of cardboard with printed screens blu-tacked on - now my sets are larger bits of cardboard and old stereo cases with printed out things blu-tacked on...
There is a large suspension of disbelief, so there are many work arounds. VCR tapes standing on their short end work as an 'industrial' wall effect, and plain old sheets of cardboard work well if you jazz tehm up a bit. Print off some door designs, then edge them with spray painted black strips of plastic sheet. The new vehicles in bubbles and six packs are a good source of the plastic - or you can use strip balsa - it's cheap and easily available.
Pringles cans, coffee tins and the like, and even metal pie tins (small ones work great as missile silo tops or sci-fi wall coverings) can be sprayed and glued onto something. Bendy straws, shampoo bottle lids, insides of earphones (the cases of 'bud' earphones make for great mini-turrets), pen caps - anything can look like something. I just thought of a suitable thing that can be a crazy invention - take a golfball, paint it black - drybrush it all over in gunmetal. mount it on a texta lid so it has a thick base, then mount that on a pedastal. Attach 4 bendy straws (painted silver or red) running from the equator , turning 90 degrees into the pedestal. Take some old bits of wire and the like to jazz it up and wahey! instant radiotropic isolatortron 9000.
I think once you have 'something to show for it' your parents will be more understanding of why you're collecting all this stuff. And if you can't do it, you can always have it off camera - I had Raptor apear in several scenes dispite the fact I didn't have his figure yet. He even shared the
true meaning of christmas in a strange pulp-fiction crossover. But anyway, I look forward to seeing what you come up with mate!