The problem with preformed pond liners is their irregular shape. It seems no one wants a square pond these days.
The big problem with plasterers baths is they don't fit in your average saloon car boot, or the back of a lot of estates. Another problem with baths is if you leave it in or on the truck, getting the beast up and into it can be a right palaver, due to its high sides and rigidity, if you take it off the truck to aid placing the carcase in you then have to get the bath plus carcase back on the truck. Not the easiest of jobs single handed especially when dealing with the bigger species of deer.
With all carcase trays you should also consider placing some form of plastic divider in the base to ensure the carcase is not laying in any blood run off.
I use large horticultural planting trays. They will take fallow, sika, and red hinds without any problem. The sides are not to high and they are easily transported and stackable, so you can take more then one with you , which means you don't have to pile carcases in on top of one anther, which aids cooling and the avoidance of cross contamination.