If you are keen to do the experiment it should be possible. The volume measurement markings will no longer be exactly calibrated but the error should be smaller than casual measuring would require.
You need to prepare 3 things
- An adjustable temperature hot air gun (or a blow torch if looks don't matter)
- A curved insulating mask (thick card stock or many thin cards) with a hole the size of one indentation (or all of them if you are a gambler)
- A hose and connection to a source of vacuum such as your mouth suction that can be fitted to bottle neck
Hook up suction, align mask, heat with 120degC hot air and use judgement. Hold vacuum until plastic cools again.