Thanks for the feedback. I'm continuing to identify options.
Option 1 - Put subwoofers in the doors. This option requires some additional work because I don't think the 40 watts available from the Haes 6-channel amp will provide sufficient power. I installed a double DIN head unit a few years ago and I could install a second amp that accepts an RCA input from the head unit. That is a bigger project than I want to tackle right now, but I could find some time in a few months. I installed an amp in my other car and it will actually be an easier job in the Boxster because the battery, amp and speaker wires are in the front trunk. If I decide on this option, I will combine it with my plan to repair the HVAC control doors. I will need to pull out the head unit for that job and running the RCA cable to the trunk will add a few minutes of work. I also need to tap into the amp control signal wire.
I haven't identified an ideal choice for a sub. There are some fairly pricey speakers that could fit but I was hoping to keep the part cost down. I had good results with Dayton Audio speakers in other cars and this sub
https://www.parts-express.com/Dayton-Audio-DCS165-4-6-1-2-Classic-Subwoofer-4-Ohm-295-198 looked like a good choice from a price perspective. But, I am not sure it will fit even with the a 3/4" adapter plate azlvr used.
Option 2 - Install a woofer that can be powered by the Haes amp. This one may work with an adapter plate.
https://www.parts-express.com/Dayton-Audio-DC160-4-6-1-2-Classic-Woofer-Speaker-295-309 I wouldn't expect the same results I could achieve with a woofer and a separate amp, but I wonder how much difference I would be able to hear while driving. During the winter when I have the hard top on my car, the audio doesn't have to compete with as much noise. But, in the summer with the soft top up or down, the audio system has to compete with so many other sound sources that I would need to spend a lot of money and time before I could hear a real difference. Based on the feedback from azlvr and Cunningr this woofer would fit with some modification to the speaker enclosure and either an mdf or plastic adapter.
Option 3 - Use a 5.25 inch woofer like this one from Crutchfield:
https://www.crutchfield.com/p_13649000/JL-Audio-C5-525cw-RP.html?tp=70205 The advantage of this is that it will drop right in, but it is pricey. Most of the folks who have upgraded also seem to think a 5.25 inch woofer has inherent disadvantages due to size that are difficult to overcome.
Option 4 - Find a woofer from a component speaker set and use it without the tweeter. This seems to be a good compromise solution. This seems to be a possibility:
https://www.crutchfield.com/p_500SS65C/Alpine-S-S65C.html?tp=78072