My DD is the same, we started TT'in on New Years day and she has picked it up really fast but she has the same issue.

We have had time when we have been out and she has had to use the potty we have in the back of that station wagon and to stop her from weeing straight forward all over the floor of the car, I now put a old style cloth nappy over her to catch the spray and then when she is done I bundle it into a plastic bag to bring home and put in the wash.

When she uses the toielt at home though we do get her to sit back as far as possible. I'm hoping that as she learns that she doesn't have to hold on so much that she will go more regularly and then avoid the excessive wees we are getting now.