Our house has no bathtubs, only showers. We have very active 17-month-old twins that love the tubs at the grandparents' and we'd love to be able to have one at home. Unfortunately all the inflatable tubs are pretty small at this point and we'd rather avoid the cost of installing an actual tub. Is there such product that's large and we can just place in the shower as is?
When my son was 2-4 years old, we had this issue and used a large plastic storage box (without the lid!) for this, and it worked very well. They're cheap (and since we had just moved had many anyway, and when he didn't need it any more, it went back into service as a box), have no sharp edges, and completely watertight. Ensure that it's the high quality strong plastic (the ones you could pack dozens of books into) and not the stuff that breaks when you barely touch it.
If I recall correctly, the one we used had a 120L capacity, and it was only when he got to 4 that it started being a bit cramped (we moved to a house with a bath at this point, so never looked into alternatives).
The worst part was that when full it's heavy, so lifting it to empty the water was something I or my wife had to do (i.e. not grandparents when visiting), although this does mean that it doesn't get knocked over and spill everywhere. There's also some care needed to empty without overflowing the shower.
If you're strong enough to carry it, there's an added advantage of any tub in that you can reuse the water (e.g. in the garden) much more simply than with a real bathtub.
We picked up a tub at BabySam here in DK for our son - he's 3 and he still fits in it perfectly:
Volume: 90 liter. Dimensions H33, B69 og L82 cm.
They also have a new sort of bathtub, one that's foldable:
Unfolded: L:67 cm, B:39 cm, H:24 cm Folded : L:67 cm, B:8 cm, H:24 cm Weight: 1,3 kg.
We just have the tub on its side against the wall of the bathroom when it's not in use; the folding ones would take up even less space.
I have all girls so I just brought them in with me when we were in this situation. I put some toys on the floor of the shower and let her sit down there and play for a couple minutes while I take care of myself, and when it's her turn I just hold her to wash her body and hair. But, once they get a little bigger their weight + slipperiness tends to cause difficulties with holding them, lol.