You have more than just one solution. The simplest and the most direct one, but the most laborious, would be to use a filter stand with a set of Cu 99.99% filters and a Unidos Romeo with a Farmer chamber.

Another solution (can be used with an horisontal beam) is to use a stationary water phantom and to measure (again, with a Unidos Romeo and a Farmer chamber) the PDD - you will be able to get the energy and thus the HVL

Another (similar solution) is to irradiate axially a Gafchromic film in a RW3 slab phantom, scan it, get the PDD, then the enegy, then the HVL

The simplest solution (but the most expensive of all), if you have a vertical beam, is to measure the PDD with a BeamScan and then get the HVL from the calculated energy.

Greetings, just wondering how accurate you do you really need this to be?

Yes the methods described above will all work and could be important in situations such as radiotherapy where the percentage depth dose of the beam for different field sizes is important in the planning systems.

A 6MeV/6MV linac beam (the two are practically interchangeable), in a unit such as a Varian Linatron (Varex Linatron) is slightly less energy than 6MV in a medical linac, but a very much greater dose rate, however its depth dose is still very similar and 50% is roughly 140-155mm in water 1g/cm^3. Being very simplistic you can now just work with ratio of densities concrete 2.35g/cm^3, 6.5cm, steel 8g/cm^3 1.9cm, lead 11.34g/cm^3 1.3cm. You can see a depth-dose here https://content.sciendo.com/configurable/contentpage/journals$002fpjmpe$002f24$002f2$002farticle-p79.xml

In the journal article it writes of the 6FFF & 6MV and has a graph showing the DD of each, the 6FFF beam is similar to what would be seen from a 6MeV Linatron would produce whilst the 6MV is the normal medical linac beam.