Struggling with fixed fee site visits


So, we have a few legacy clients where we do fixed fee on site work - 1 or 10 hours, same fee (+ Expenses).

I really want to get this all working automatically if there is a scheduled "onsite" Work Role.

I've tried it via modifying Work Roles and Work Types and got very close by setting min/max hours to 1, however, it doesn't then show the correct time on the invoice.

After chatting with support, I've been told of a few workarounds:

1. Multiple fixed fee tickets per visit (although, with fixed fee, I need to manually calculate expenses, and again, manual process).

2. Have an on site visit product and add this to the ticket (Will work - and considering it... but, just doesn't feel as neat as it should be).

I was just wondering how other people would handle this situation?