For moulds, I use the plastic storage boxes you get from Go-Lo, they start at about $5 depending on the size. For smaller snakes, the decor 5 litre red lunchboxes work well. You can get big ones - about 15-20 litre for under $10.
For my lizards I have also used 'In-trays' ... the plastic ones that are stackable that you find on any office desk. No need to make an entrance hole for these.
Melt an entrance to the container with a soldering iron or cut with a stanley knife. I find the soldering iron better because sometimes the plastic can be brittle and stan can snap it. If you use the soldering iron do it in a well ventilated area as the plastic will give off fumes as it melts.
Cover with expanda foam .... wear disposable gloves! ..... pat down lightly as you go to so it smooths out a little and doesn't look like long strands of turds!
Leave for 24 hours and then paint. Don't bother with grout or cement on expanda foam because it doesn't stick very well (even with bondcrete in it). I use cheap fence paint in mission brown or green as a base and then highlight with cheap tubes of acrylic paint from the $2 shop.
I find sponges - the ones with the harsh green side and the yellow sponge bit - are great for applying paint affects, and spray paint works a treat as well.
One large can of expanda foam does one large hide (15 litre container) and one small hide (5 litre lunchbox) and an In-Tray.
I'm just starting out but having a great time making stuff, and I'm getting a bit carried away getting foam from dumpsters outside shops!
I've done a couple of backgrounds, some hides and just some random formations for my lizards to climb under or over.
I still use alot of natural things like real rocks and branches, but made stuff is much easier to clean.
P.S. Old style foam eskies also make a great base for a hide for smaller snakes & lizards