Where are you going to use one?
That makes all the difference.
If you are going to use one behind a bush , just 'wild wee'. I do that on country walks but make sure you pull your knickers out of the direction of the flow.
If it's in a loo (but not sitting on the seat) I'd just carry a plastic container like a drinks bottle with a wide opening at the top and wee in that then pour it in the loo.
I have practised using a Travel John in the car and it was impossible to wee in it sitting down (answers on a p card please for any tips.)
My criticism is that the bit you put next to your body is not very big (and I think my under carriage is average.)