Look for a sand mix (deck mud). This is the same stuff used to build a shower pan. It is often prepackaged with 3 parts sand to one part portland cement (or you could mix your own - more work, but you won't need all that much). Or, you could use Structolite (sp?). A sand mix is close to wet beach sand - it doesn't flow. It should only be mixed with enough water to hold together when you grab and squeeze a clump (and not drip water). It will cure to a solid mass, but be grainy - you could scratch it easily. It will pack down when you set the tub into it so you want it a little deep, then settle the tub down into it so that it sits level. If the floor isn't perfectly level, and you've got good coverage, it doesn't need to have the legs sitting on the subfloor or slab (but, you need good coverage!). It's great in compression - probably a couple thousand pounds per square inch. You could put a sheet of plastic both under and over it - this will ensure the moisture doesn't evaporate, and it chemically cures which increases the strength. When cement cures, it incorporates the water into its crystaline structure - it doesn't dry. You mix a sand mix (deck mud) with a hoe and a rake. They make a special one just for this, but for one batch it's overkill. Mix it up real well before you add the water, and then add water a little at a time until you get it to clump when you grab a handfull and squeeze. It all needs to be damp, not soaking, and well mixed. You'll have probably 45-minutes after you start to wet it until you want to have everything in place, maybe longer if it is cool.
You could mix this in the bathroom on the floor if you put down a tarp or some roofing felt or some plastic. You want something you can roll up and then dispose of. But, carrying a few big buckets of the stuff you've mixed say in the driveway works well, too. Just don't track it in on the carpet along the way...cement is a bear to get out and the sand could scratch flooring along the way if your shoes aren't clean.
