The Quarq DFour DUB Power Meter is indeed designed for users of a Shimano crankset that want to add a power meter. The DFour uses a 110 BCD, 4-bolt spider so you can use your Shimano chainrings on it. (The power meter is designed for use with Shimano DURA-ACE 9100 chainrings, however Ultegra R8000 and 105 R7000 rings also work.)
However, please note that you must replace your entire crankset with the new DFour crankset (power meter spider plus new Quarq DZero crank arms). This is because Shimano road cranksets have an integrated right-side crank arm and spider (the spider and crank arm are one solid piece), so you cannot physically install just the DFour Power Meter Spider on your Shimano crankset. Rather, you must purchase the DFour power meter spider plus the Quarq DZero DUB Crank Arms.
On our product page for the DFour power meter, you can easily bundle the power meter with crankarms by simply selecting the crank arm length that you need. Please note, you also need a SRAM DUB Bottom Bracket.
If changing your bottom bracket sounds like more work than you would like to go through, we have a great alternative.
Like the Quarq DFour, the power2max is a new crankset (power meter spider and crank arms). However, with the power2max, you don’t have to change your bottom bracket. You can order the power2max in the same 24 mm spindle size your Shimano crankset uses making it a direct replacement for your Shimano crankset. In addition, as with the DFour, you can use your current Shimano chainrings on it (just be sure to select the 110 4-Bolt Shimano BCD option). power2max makes wonderful power meters. They measure both left and right leg power, are reasonably priced and are as good as it gets in terms of reliability.