1. Cardboard is OK. Cover it in plastic. Support large areas of cardboard or they will bend under the weight. Small boxes can support themselves but all cardboard loses strength when it gets wet.

2. Castors should be set into a reinforced structure or they will be prone to breaking off the corner of your planter. You could just put the planter on a small wheeled tray, or even build the whole tray into the base of the planter. Think about the weight. A 3'x2' planter will be too heavy to lift even when empty.

3. Curing time depends on temperature. At least a week if it is very warm. For maximum strength, cure it for a month in cool conditions. The surface should not be allowed to dry out during the cure. For most plants, you should also leach away lime created at the surface by the cement.

So good of you to consider giving up virginity in favour of addiction!
Once you get your first cast out of the mold,and the wet smell of setting concrete in your nose,you're hooked.And,as a famous Canadian author and humourist once said,you'll"fly madly off in all directions."Hypertufa,lightweight products,sandcasting,making your own molds will be begging you for attention.Family and friends,seeing this addiction,will try to have an intervention.Too late though.Once this hobby gets its cold,wet hooks into you,it'll be with you forever.
Be afraid-be very afraid.

Cheers from here

I too am a hypertufa virgin, have been lurking here for some time trying to get up the courage to get started. I have done leaf castings and stepping stones with cement but am really curious about the hypertufa, the info and ideas on this site are so inspiring that I can't resist the urge to "play in the mud". Anyone in the Langley B.C. area need a student?